Fig.14 Isometric crystal of analcime (Photo by John Bett)
Fig. 15 Elongated crystals of tourmaline (Photo by John Bett)
Fig. 16 Flattened crystals of hematite (Photo by John Bett)
2. How a mineral resists scratching is the measure of its hardness. A mineral will resist scratching or demonstrate its hardness relative to the substance used to scratch it or the substance that the mineral can scratch. German mineralogist Friedrich Mohs developed a hardness scale involving ten minerals from the softest to the hardest during the 19th century. His scale,
known as Mohs hardness is the standard by which mineral hardness is determined. Only fresh mineral surfaces should be tested for hardness.
| Hardness | Mineral | Test |
| 1 | Talc | Can be scratched by a fingernail |
| 2 | Gypsum | Can be scratched by a fingernail |
| 3 | Calcite | Can be scratched by a penny |
| 4 | Fluorite | Can be scratched by a glass plate |
| 5 | Apatite | |
| 6 | Orthoclase | Can be scratched by a knife blade |
| 7 | Quartz | |
| 8 | Topaz | Can be scratched by a steel file |
| 9 | Corundum | |
| 10 | Diamond | Hardest known mineral |
Table 3 Mohs Hardness Scale
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3. Color is a physical property of limited value in mineral identification. For example, quartz can be of nearly any color because of slight mineral impurities. Quartz can be cloudy, white, pink, purple, red, black, smoky, orange and brown. Color is not usually sufficient by itself to identify a mineral; however, in combination with other physical properties it can assist in mineral identification.
4. The streak of a mineral is its color in the powered form. Streak is obtained by drawing the mineral across an unglazed porcelain plate. In many minerals the streak is more diagnostic than color and is often very different from the mineral color. Minerals that are harder than the porcelain plate produce no streak.
5. Minerals have different weights relative to each other. Some are relatively heavy and others are light. Specific gravity of a mineral is its weight relative to the weight of an unequal volume of water. It is obtained by dividing the
weight of the mineral in air by the difference between its weight in air and its weight in water. Most minerals specimens are difficult to measure for specific gravity because they are usually associated with other minerals. However, very heavy or very light minerals are most easily recognized by specific gravity.
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6. How a mineral reflects light determines its type of luster. Most minerals are considered to have metallic or nonmetallic luster, with a few having a submetallic luster.
Fig. 17 Branching formation of bright metallic gold crystals (Photo by John Bett)
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