Color is one of the properties we notice first. The color of some minerals is always the same, and it helps us to identify them. But it is not a dependable property to use in identifying all minerals, because some contain impurities that change or hide the real color.
Luster
The luster is the way the surface of a mineral reflects light. The luster of a mineral may be nonmetallic, submetallic, or metallic. Mineral metals such as [gold], silver, [galena], and [pyrite] have a metallic luster. A few minerals have a luster that is almost, but not quite metallic—their luster is submetallic. A mineral with a nonmetallic luster may look vitreous (glassy), silky, resinous (like resin), greasy, earthy (dull), pearly, or adamantine (brilliant).
Transmission of Light
Some minerals allow light to pass through them; others do not. A mineral is [transparent] if you can see both light and objects through it, as through clear glass. If you can see only light, but no objects, as through frosted glass, the mineral is [translucent]. When you hold an [opaque] mineral up to the light, it looks dark. No light at all comes through it, even through the thin edges.
[Transparent] mineral.
Hardness
Some minerals are soft and can be scratched easily. Others, which are harder, are resistant to scratching. To measure a mineral’s hardness, we try to find out which substances will scratch it and which substances will not scratch it. To do this in a general way, several ordinary objects—such as a fingernail, a copper penny, a pocket knife, a piece of window glass, and a steel file—can be used. For a more exact way of testing hardness, we can use ten minerals that make up what is known as Mohs scale. Each mineral in this scale has a different hardness, and each one has been given a number that represents its hardness. For example, [talc], the softest mineral in this scale, is given a hardness of 1. [Gypsum], the next softest mineral in the scale, has a hardness of 2. Diamond, the hardest mineral known, is given the top hardness of 10 in this scale. These ten minerals are listed below. Alongside them are five common objects with their hardnesses.
| 1—[Talc] | |
| 2—[Gypsum] | Fingernail—slightly over 2 |
| 3—[Calcite] | Copper penny—about 3 |
| 4—[Fluorite] | |
| 5—Apatite | Pocket knife—slightly over 5 |
| 6—[Orthoclase] | Window glass—5½ |
| 7—[Quartz] | Steel file—about 6½ |
| 8—[Topaz] | |
| 9—Corundum | |
| 10—Diamond |