1. Introduction
Hard rock mining is not concerned with industrial minerals, salt, or coal, but almost always with metallic ores. (An exception is mining gemstones from pegmatite dykes or the harder volcanic pipes.) The delineation and grade of a hard rock ore body are first made by diamond drilling (as opposed to reverse circulation drilling or test pits). Confirmation (risk reduction) is accomplished if miners subsequently drive an underground entry to complete additional exploration drilling and sampling.
Miners use the term “exploration geology” to denote the implementation and interpretation of all this work. This chapter is mainly concerned with the sequence of events that occur between the discovery of a hard rock ore body (mineral reserve) and the determination that it constitutes an ore reserve (mineral reserve that may be exploited at a profit).
In recent years, the miner’s role has been less concerned with exploration geology and ore reserve estimation. The latter, once the domain of the mining engineer is now typically performed by “geoscientists” (geologists and geo-physicists). The change has not occurred without problems. The root cause is commonly believed to be that geo-scientists were not regulated by a professional body and hence could not be held legally responsible for their actions. As a remedy, some Canadian provinces (with the notable exception of Ontario) and a few of the states are now admitting qualified geo-scientists into their associations of professional engineers.
Hence, this chapter does not provide full consideration for geology and reserves, but deals more with driving entries for underground exploration, which is the domain of the miner.
2. Rules of Thumb
Discovery
• It takes 25,000 claims staked to find 500 worth diamond drilling to find one mine. Source: Lorne Ames
• On average, the time between discovery and actual start of construction of a base metal mine is 10 years; it is less for a precious metal mine. Source: J.P. Albers
Costs
• The amount expended on diamond drilling and exploration development for the purposes of measuring mineral reserves should approximately equal 2% of the gross value of the metals in the reserves. Source: Joe Gerden
Bulk Sample
• The minimum size of a bulk sample, when required for a proposed major open pit mine, is in the order of 50,000 tons (with a pilot mill on site). For a proposed underground mine, it is typically only 5,000 tons. Source: Jack de la Vergne
Ore Resource Estimate
• The value reported for the specific gravity (SG) of an ore sample on a metallurgical test report is approximately 20% higher than the correct value to be employed in the resource tonnage calculation. Source: Jack de la Vergne
• To determine “inferred” or “possible” reserves, it is practice to assume that the ore will extend to a distance at least equal to half the strike length at the bottom of measured reserves. Another rule is that the largest horizontal cross section of an ore body is half way between its top and bottom. Source: H. E. McKinstry
• In the base metal mines of Peru and the Canadian Shield, often a zonal mineralogy is found indicating depth. At the top of the ore body sphalerite and galena predominate. Near middepth, chalcopyrite becomes significant and pyrite appears. At the bottom, pyrite, and magnetite displace the ore. Source: H. E. McKinstry
• In gold mines, the amount of silver that accompanies the gold may be an indicator of depth. Shallow gold deposits usually have relatively high silver content while those that run deep have hardly any. Source: James B. Redpath
• As a rule of thumb, I use that 2P (Probable) reserves are only such when drill spacing does not exceed five to seven smallest mining units (SMU). Open pit mining on 15m benches could have an SMU of 15m by 15m by 15m. Underground, an SMU would be say 3m by 3m by 3m (a drift round). Source: René Marion
Strike and Dip
• The convention for establishing strike and dip is always the Right Hand Rule. With right hand palm up, open and extended, point the thumb in the down-dip direction and the fingertips provide the strike direction. Source: Mike Neumann
3. Tricks of the Trade
• A likely location for hard rock discovery is near a major intrusion of a coarse grained igneous rock into another that is fine grained. Discoveries are made within two kilometers of the contact, but not within the intrusive. Source: Northern Miner Press.
Ore bodies with a hydrothermal genesis are often found near the nose of a fold. Folds are identified on maps published by a national geological survey. Source: Northern Miner Press.
• Metamorphosed sedimentary piles of argillites, black schist, and calcareous rocks are particularly favorable to a rich gold discovery. Source: R. W. Boyle
• Classic volcanic massive sulfide deposits typically occur in districts having a diameter of approximately 32-km. On average, each district contains 12 deposits that all together may contain a total in the order of 5 million tons of metal. Ranked in order of size, the largest deposit typically contains 2/3 of the total metal and the second largest about 1/7. Source: Dr. D.F. Sangster
• Prevailing regional cross folding indicates the direction in which a new discovery may be found in the vicinity of one already identified. For example, in the Canadian Shield a new discovery is most likely to be found to the northeast or southwest of an existing ore body. Source: S. V. Burr
• An ore body may have been split and separated by post-mineral dykes, sills, or slip faults. Determining the displacement may permit discovery of the “other half” of an ore body. Source: Arturo Thomas Novoa
• If you spend a ton of money on underground development at the exploration stage, you can wind up by making a mine out of a project that would otherwise have failed (when it is considered that sunk costs are often not considered in the final economic feasibility study). Source: Bill James
• Exploration diamond drilling from surface should penetrate past the zone of mineralization and into the footwall where future mine development may take place. Source: Jack de la Vergne
• An “ore reserve” calculated only from surface drilling is typically based on measurement in two dimensions; the third is assumed. Measurement in three dimensions is required to determine an ore reserve. Source: Jack de la Vergne
• Estimation of ore reserves and grades from sample assays based on simple arithmetical averages leads to fundamental error; statistical averaging is required. On the other hand, even sound statistical procedures (e.g. geostatics) when employed by gifted amateurs can do more harm than good. Source: Jack de la Vergne
4. Diamond Drilling
The hard rock ore bodies that outcropped on surface were found long ago. Diamond drilling enables the discovery of new ore bodies that outcrop beneath deep soil overburden, under lakes and even those that do not outcrop at all (blind ore bodies).
The role of the diamond drill is not completed with discovery. In hard rock mines, it is the weapon of choice for subsequent exploration drilling required to define the ore body. Later, diamond drilling is employed for definition drilling to locally define an ore outline before actual mining. longitudinally (½ for assay and ½ permanently retained in storage). Larger diameter cores may be split again to provide separate samples for assay and metallurgical testing. The drilling process returns cuttings (sludge) to the surface to supplement the core assays.
The diamond drill core may be obtained in any one of a number of standard diameters. Larger diameters are preferable but more expensive. The most common sizes employed today for exploration drilling are NQ and CHD 76. Core and drill hole diameters for various bit sizes are
provided in Table 1-1.
Drilling logs provide indices of the rock qualities, including penetration rate, core loss (an indicator of bad ground), loss of return water (indication of a potential inflow of groundwater to future mining), grout take, etc. In addition, all core extracted is separately logged at a core shack. Particular attention is paid to the sections that penetrate the ore and that portion of the footwall where future mine development may take place. Geologists equipped with a bi-polar microscope and portable computer (laptop) perform the logging. The PC software includes a menu of formats, check lists, symbols, abbreviations, and repetitive terminology.
Exploration drilling is performed from surface with holes laid out on a prescribed grid (pattern). The grid orientation conforms to the dip or plunge of the ore body. Grid coordinates are tied to the federal (national) grid system. Collar elevations for each hole are best determined from a benchmark(s) tied to the nearest national geodetic monument. The spacing between holes on the grid is established from the presumed orientation and extent of the ore body. It is convenient to use the same units of dimension (metric or imperial) for the grid spacing as used in the national grid of the host country. A spacing of 200 feet by 200 feet (60m by 60m) is commonly employed.
If the ore body is flat lying or slightly inclined, it is convenient to drill all holes vertically. Parallel, inclined holes are drilled for steeply dipping ore bodies. All drill holes will deviate from their planned trajectory. The deviation is measured at intervals with down-the-hole devices, none of which are precise. For deep drilling, the rig is equipped to re-align the drill string and keep it on line. In the past, it was considered good practice to fill the empty borehole with cement grout when completed. Today, this procedure is delayed because an open hole (even if it encountered no mineralization) is valuable for the purpose of cross-hole tomography, etc. that may indicate an ore zone missed by the drilling pattern.
5. Evaluating Exploration Properties
Properties with fully developed ore reserves are evaluated for potential as a profitable mining enterprise by formal procedures. (Refer to Chapter 6, Feasibility Studies, and 7, Mineral Economics.) Frequently, properties are required to be evaluated (for sale, joint ventures, or other transactions) with only drill indicated or inferred reserves. Sometimes, untouched exploration properties are valuable simply because they occur in a fashionable area, such as near a recent spectacular discovery.
If exploration expenditures are incurred on the property, expenses not resulting in condemnation enhance property value. If no indication of mineral resource has been identified and the property is idle, the value must be limited to no more than half of the spent costs. When results are positive, exploration is ongoing, and work to date has been completed with diligence and efficiency, 100% of the funds already expended can be added to the property value.
A different approach was originally developed to assist with properties submitted for approval by the Toronto Stock Exchange. The method assumes that a property is first acquired by staking claims, the cost of which is known. The first cost is multiplied by weighed factors for items of value (such as regional geology, proximity to infrastructure, geological data quality, executive integrity, or field manager reputation) to obtain valuation.
Other approaches exist to evaluate mineral properties, including so many dollars per ounce of “gold in the ground,” but these methods are no longer popular. Liabilities must be subtracted from the positive values of an exploration property. For example, the purchaser or partner may become responsible for the cost of clean up and restoration if the property is later abandoned. Liabilities are not normally significant for a green field play, but if the property is environmentally sensitive; subject to native land claims; or contains old dumps, tailings, or mine workings, it is prudent to assess the liabilities.
6. Estimating Ore Reserves
The ore reserve is the mineral resource that may be extracted at a profit. A more precise definition is, “that part of a mineral resource that has been analytically demonstrated to justify mining, taking into account, at the time of determination, mining, metallurgical, marketing, legal, environmental, social, economic and other applicable conditions.” (Extracted from the Johannesburg Stock Exchange listing requirements: Mineral Companies, Chapter 12.) Estimation includes determining tons, grade, and degree of certainty (proven, probable, or possible).
Tons
Resource tonnage is obtained by multiplying ore volume by its density. For example, 1,000 cubic meters of ore with a specific gravity (SG) of 3.0 weighs 3,000 metric tons (tonnes). The volume is computed from ore outlines and the specific gravity determined by weighing a sample in air and suspended in water. The calculation of volume is not complicated and may be determined with confidence, provided the ore outlines are accurate. Unfortunately, less attention is given to the accuracy of specific gravity. Sources of the figure(s) provided should be questioned.
A wrong value is obtained from slurry analysis carried out in a metallurgical testing laboratory. The reason is that ore is porous and when finely ground, the density of individual particles is approximately 20% higher than the density of a block of ore.
In a ‘minable ore reserve’ calculation, resource tons are “reduced” to account for the fact that not all the ore will be mined. Conversely, resource tons are “increased” to account for dilution with waste rock in a contact ore body or with low-grade material in a cut-off ore body. Nineteen different contributing factors are considered in a comprehensive estimate of the amount of dilution (refer to Chapter 3 – Mining Methods.)
Ore reserve tons are categorized into those in which the dimensions used to obtain volume were actually measured and those based on assumed measurements. Measured tons are divided into proven and probable reserves. Definitions and guidelines for these terms abound, but they are imprecise. The one exception is that for a contact vein deposit, proven reserves are those measured on at least three sides of a block, no side of which exceeds 60m (200 feet) in length.
The probability of proven reserve tons being accurate is 100%, probable reserves 80%, and possible reserves 50%. (National Mining Code of Chile)
Grade
Proper grade determination for an ore body is difficult and time consuming. “The arithmetic mean is a very inadequate axiom. Instead of adding up a series of observations and then dividing the sum by the total number of observations, equal suppositions would have equal consideration if the estimates were multiplied together instead of added. Mother Nature is not troubled by difficulties of analysis, nor should we.” Lord Keynes Elementary components (observations) consist of ore body sample grades and location. In hard rock formations, these typically consist of assay results from diamond drill cuttings (sludge), split drill core, and channel samples. Sometimes these are augmented by bulk sample assays or cuttings from inclined percussion drilling into the walls of exploration headings. For the sample assay grade to be correct, they must be collected properly and protected from contamination (or salting) in transit. Except for a major mining company with in-house expertise, a recognized independent laboratory should perform the assays. The best-recognized laboratory available should be selected to perform periodic check assays. For foreign projects, all assays, or at least check assays, should be performed domestically.
Note
Problems may arise when shipping sample bags to the home country unless they are double tagged (one may be torn off by baggage handlers) and clearly labeled, “Pure mineral rock samples” to avoid detainment in customs. Once samples are taken, ore reserves are divided into blocks of convenient size. A grade for each block is determined from samples in and near the block. Each sample assay used for the block grade determination is assigned a weight. The sum of the weights is one (or 100%). Weights are dependent on the degree of variation between the samples employed; grade resolution is determined by the application of statistical analysis to the variations. A geostatical tool, “Variogram,” is typically used to represent the variance of samples with respect to the distance separating them. The block grade is determined by summing the products obtained from multiplying each sample grade by its assigned weight.
Blocks not meeting the cut-off grade are removed from the reserve ton calculation. The cut-off grade is traditionally the breakeven point (neither profit nor loss). Recently, cut-off grade has been chosen to ensure a low cost product compared with the cost incurred at competing mines around the world. When the mine is in production, the cut-off grade may be lowered after the preproduction capital cost has been retired. Cut-off grade may be raised or lowered at any time during mine life depending on prevailing metal prices.
7. Underground Exploration Entries
The nature and circumstance of the typical hard rock ore deposit is such that the exploration program may not properly be completed without including exploration work from an underground entry. This concerns open pit projects where a representative bulk sample is required (Twin Buttes, Tyrone, Brenda, Endako, Marcopper, Palabora, Escondida); and especially pertains to underground deposits. Numerous instances have occurred where underground mines developed without such a program, encountered significant problems due to unforeseen circumstances.
Those mines successfully brought into production without an exploration entry typically involved ore bodies clearly defined from surface drilling because of the nature of the mineralization and/or the proximity of very similar deposits already mined. Even these circumstances are not foolproof.
For example, the Randfontein mine (where the ore is exceptionally uniform and continuous) encountered an unexpected barren area that interrupted the ore throughout a horizontal length of over 8,000 feet along the reef. Industry standards and good engineering practice normally require that a hard rock mining project begin with an underground exploration program before proceeding with a definitive (bankable) feasibility study.
Below ground only can the miner ‘shake hands with the ore.’ Arnold Hoffman, 1947 Listed below are the specific reasons for completing an underground exploration program.
• Confirm existing ore reserves
• Define the ore body
• Obtain geotechnical data
• Obtain a bulk sample
• Test mining methods
• Measure ground water flows
• Further exploration
• Confirm Existing Ore Reserves
Surface drilling permits measurement of the ore reserves from only two dimensions. Hence, none of the underground mineral deposit can be officially classified as “proven.” Three-dimensional measurement may be only undertaken from underground to confirm continuity of ore outlines between drill holes.
“When it comes to measuring ore reserves accurately, the key is a proper mix of sampling theory (statistics) and geology. Geostatical methods depend heavily on large sample numbers and extensive close-spaced sampling, including heavily drilling local areas to estimate mining selectivity. Extensive drilling may not be economical in a small ore body. Even in a large ore body, going underground may ultimately be the only way to determine how well the ore can be followed.” Gary Raymond, Canadian Mining Journal, August, 1985
Define the Ore Body
Most surface drilling requires substantial distance to reach the underground ore deposits. The distance and length of drill string required can result in considerable hole deviation. The deviations can be determined and considered, but the measurement is not always accurate. Inaccuracy can result in a distorted interpretation (whether by computer or manual means) of the actual ore configurations, outlines, continuity, fault lines, and grade distribution.
For mining engineers to select the appropriate mining methods to permit the safe and economical extraction of the largest possible percentage of identified reserves, reliable and definitive information on grades and widths is required. This requirement can only be met by going underground.
Obtain Geotechnical Data
Ordinarily, geotechnical/rock mechanics data can be obtained from the drill core and logs gathered during surface drilling. However, early drilling is often completed to identify ore and mineralization grades to confirm the general project direction and/or "sell" the project. Typically, little or no consideration is given to geotechnical properties, some of which can only be measured accurately from freshly extracted core. Core that could provide geotechnical data often is consumed for assay and bench testing purposes or is kept for verification purposes.
Some required geotechnical information (measuring the direction and magnitude of the ground stress regime) can only be completed underground. An underground exploration development program should provide reliable values for ground stress as well as unconfined compressive strength (UCS), modulus of elasticity (E), specific gravity (SG), work index (Wi), internal angle of friction (f), and bulk density of the broken ore.
Together with values for rock quality designation (RQD), joint indices (J), and stress reduction factor (SRF) obtained from proper drill core logs, this data describes the engineering properties of rocks to be developed, supported, built against, and mined. The resulting array of geotechnical criteria is essential for sound underground mine design.
Obtain a Bulk Sample
For proper metallurgical testing, large sized samples are required (much larger than can be obtained from drill core). Only from underground can representative ore samples be obtained in the quantities required. Bulk samples are especially important when bench testing (on drill core) indicates a complex metallurgy requiring significant testing and analysis to obtain a high percentage of mineral recovery in the process plants (i.e. mill, smelter, and refinery).
In addition, bulk sampling enables advance determination of whether preventive measures are desired to reduce detrimental oxidation of wall rock and/or broken ore resulting from an undesirable mineral component such as pyrrhotite. The bulk sample will also enable further confirmation of ore distribution and grades.
Test Mining Methods
Conventional practice requires excavating test stopes underground to obtain the bulk samples described above. Examining these larger sized openings is valuable in evaluating mining methods for the ore body. Ground reinforcement required to maintain the structural integrity of the excavations (rock bolts, screen, etc.) can be monitored for long-term stability. The results may be later applied to establish safe ground support criteria and standards for underground operations.
Measure Ground Water Flows
The methods used to predict water inflows underground from surface drill holes (packer tests) are inadequate for the accurate measurements required to determine the underground pumping and dewatering requirements of a hard rock mine. Only by going underground can the requirements for grouting and de-watering be reliably determined in advance.
Further Exploration
An underground exploration program is typically designed to uncover additional ore extensions and satellite zones of mineralization that may have been missed by surface drilling.
8. Mine Entry Comparisons
The following paragraphs compare early mine entries for underground exploration. For hard rock mines, different suitable entry types exist, each representing a substantial effort and expenditure. The type and location of an exploration entry must be selected with diligence to ensure the entry will not interfere with future development, and to determine potential value as a permanent facility for subsequent development and operation.
Exploration entry design requires consideration of the permanent production facility (Conceptual Mine Plan), particularly with respect to material and personnel handling as well as the permanent ventilation circuit.
Listed below are the entry types used for underground exploration.
• Vertical Shaft
- Rectangular timber two compartment
- Rectangular timber three compartment
- Circular concrete lined (monolithic)
- Circular concrete lined (concrete rings)
- Circular bald
- Drilled shaft
• Inclined Shaft
• Trackless Entry
- Ramp Access [suitable for load-haul-dump (LHD) equipment access)
- Decline Access (suitable for a belt conveyor installation)
- Adit Access (suitable for either – in mountainous or foothill regions of high relief)
• Double Entry
Vertical Shaft – Rectangular Timber Two Compartment
The rectangular timber two-compartment shaft is suitable for remote locations because the sinking plant is relatively small and the components are typically provided in portable modular or prefabricated units. The disadvantage is that these shafts are not suitable for depths greater than approximately 400m and they do not have the hoisting capacity required for subsequent development of a large underground mine. The sinking hoist is small and hoists only a single line in one of the two compartments (the other is required for the manway, vent duct, and service lines).
Vertical Shaft – Rectangular Timber Three Compartment
The rectangular timber three-compartment shaft is the most common type of entry employed for an underground exploration program. Although more expensive, the shaft can be sunk to great depths and have a much higher hoisting capacity due to the larger plant and ability to hoist with two skips in balance. When the exploration and mine development is completed, this shaft can be used as second egress from underground or stripped of its timber (and slashed, if necessary) to provide a permanent ventilation entry.
The three-compartment shaft is the conventional choice for an initial entry. In some applications, timber sets have been replaced with steel. Steel is lighter to transport but less adaptable for installing catch pits and water rings. Steel sets are not recommended unless the sets must be shipped to the site by air. This type of shaft could be suitable for an ore deposit requiring extensive lateral development in the exploration phase and is too deep for ramp haulage during the subsequent preproduction development phase.
Vertical Shaft – Circular Concrete Lined (Monolithic)
The circular concrete lined (monolithic) shaft is not commonly employed as an exploration entry unless the anticipated ground conditions are poor or the shaft is very deep. Advantages include shaft sinking at a faster rate than timber shafts, a high hoisting capacity and future service as a major entry for the permanent mine with little or no modification. A disadvantage is that these shafts are expensive and slow to set up for sinking. Another problem is that the exact diameter to fit the permanent mine entry must be determined in advance, which can be difficult or impossible. A specific disadvantage at a remote location is the requirement for significant quantities of concrete. This can be difficult and expensive during the early project stages.
Vertical Shaft – Circular Concrete Lined (Concrete Rings)
The circular concrete lined shaft with concrete rings employs a segmented concrete lining. Horizontal segments or "rings" approximately 1.5m in height are poured against the wall of the excavation at approximately 5m centers. The open ground between the rings is permanently secured with rock bolts and screen. Concrete ring shafts can be less expensive than monolithic concrete lined shafts. One disadvantage is slow sinking compared to the monolithically designed shaft. Another is the concrete rings becoming a problem (high resistance factor) if the shaft is subsequently employed as a high velocity ventilation airway.
Vertical Shaft – Circular Bald
Instead of using concrete, the circular bald shaft walls are secured entirely with rock bolts and screen as the shaft advances. While relatively inexpensive compared to concrete, it is generally considered less safe to sink. While often employed in developing countries, circular bald shafts are rarely found in North America. One sunk a few years ago in Northern Manitoba is reported to have developed problems since completion. Some of the difficulties with this type of shaft include the lack of catch pits, challenge in installing water rings, and difficulty in placing working platforms in the shaft. Circular bald shafts typically employ rope guides and thus have no sets.
Vertical Shaft – Drilled
The drilled shaft is typically the most expensive and is normally employed only where the shaft must advance through a horizon of very poor and/or heavily water bearing ground. These shafts are often advanced very quickly but only after a prolonged set-up period. An apparent exception is shafts that have been drilled in “good” ground in Australia.
Inclined Shaft
The inclined shaft is rarely used except to access undersea deposits. A number are operating in Cape Breton, Nova Scotia. Disadvantages include the relatively slow rate of advance and limited hoisting capacity for subsequent operations. For example, four inclined shafts were required to hoist the iron ore on Belle Island, NFLD.
Trackless Entry Ramp Access
Ramp access is the usual choice for shallow ore bodies, particularly when the ore is flatly dipping. Even if the underground deposit is steeply dipping, permanent ramp access from surface is desirable for an operating mine. Providing a ramp cross-section large enough to accommodate truck haulage should be considered. If not, at least the portal should be built oversize. Since the ramp is flexible with respect to alignment, the portal can be located where the overburden is shallow or a surface outcrop is encountered.
Trackless Entry Decline Access
The decline access is similar to the ramp access except that it must be driven straight to accommodate a belt conveyor. For a very large ore body, it is often the case that the only practical alternative to a permanent conveyorway is vertical shaft hoisting. Ore is normally conveyed on surface from a shaft to the mill (if a considerable distance). Therefore, it is practical and logical to drive the first leg of a conveyorway toward a potential shaft location because it permits an underground dump in the shaft.
If the shaft option is later selected over a conveyorway, the skips can dump underground instead of on surface. Ore can be conveyed directly from the underground dump to the mill by the top leg of (what would have been) a full conveyor system. Advantages include providing a route for alternate egress, LHD entry, a lower headframe, and less real estate on surface (no overland conveyorway).
This procedure is employed at the Shebandowan and Dome mines in Ontario, Canada. The trackless entry decline is a practical alternative if the portal location lies in shallow overburden and the production shaft location can be determined in advance.
Trackless Entry Adit Access
Adit access is advantageous in high-relief terrain. Similar to a ramp or decline, if driven beneath the ore body, it can serve as a future haulage way or drainage tunnel.
Double Entry
A single entry will not provide the ventilation later required to support the rapid advance of preproduction development excavation for a major underground ore body. Proper ventilation must be provided by a second entry. One option is to advance simultaneously two separate entries into the ore deposit at the exploration phase.
The main advantage of this option is the time it will save in the subsequent pre-production phase. It also provides second egress (escape route) at an early stage. If either entry is slowed by unforeseen events (encountering water or adverse ground), the exploration schedule will not be compromised. Implementing this approach at the exploration stage provides a head start to production with a high degree of reliability.
9. Tables
Listed below are tables appended to this chapter.
• Geologic Time Table (Abbreviated)
• Mineral Hardness
• Rock Hardness (Typical Values)
• Geologic Time Table (Abbreviated)
Table 1-2 shows chronological, top-down geologic time.
Mineral Hardness
Table 1-3 shows Mohs’ Scale for mineral hardness.
To test a new specimen, a smooth surface is selected, and rubbed with a tool or standard specimen. A fingernail will scratch to a hardness of 2 ½, copper penny 3, and penknife 5 ½. In general, a scratch will appear when a standard mineral of equal or greater harness is used.
Rock Hardness
The following table shows Mohs’ Scale for rock hardness
