When coming across a rock, it may be difficult to determine what minerals, if any, make up the specimen. Identification is made easier if the characteristics of minerals areknown. These characteristics include color, luster, streak, cleavage, hardness, specific gravity and chemical composition.
It is difficult to use color when identifying a specific mineral. One mineral may present itself in a variety of colors, or they may be seen in an unexpected color if a chemical has been added to them. Another process that can change the color of a mineral is weathering that the mineral may have been exposed to. Quartz is an example of a mineral that comes in many different colors. It could be rose, milky or clear.
Another characteristic used to identify minerals is luster. This is the way the mineral reflects light. When describing a mineral’s luster words often used include: metallic, pearly, glassy, silky, greasy, brilliant or dull. Some examples of minerals and their luster are: diamonds have a brilliant luster, talc is greasy, quartz is described as glassy and pyrite, or fool’s gold, has a metallic luster. When discussing luster, some of the words that are used are dull, waxy, resinous, sub-metallic and so on.
When a mineral is rubbed across a hard, rough surface, if it leaves a color, this is called a streak. When pyrite, or fool’s gold, is rubbed on a rough surface, it leaves a greenish-black streak. Field guides are handy to identify a mineral by its streak.
Cleavage describes the way a mineral breaks. Galena cleaves into cubes, but mica cleaves in thin sheets.
The ease that it takes to scratch a mineral is its hardness. Friedrich Mohs, a German mineralogist,developed a hardness scale in 1822. Talc, a soft mineral, can be scratched easily with a fingernail; this is given a hardness number of 2.2. If a mineral can be scratched by a copper penny it is a 3.5; by a pocket knife, a 5.2; a piece of glass, a 5.5; a steel file, a 7.5; or a piece of corundum, a 9.
The measure of a density of a mineral is determined by specific gravity. This is done by comparing the weight of the mineral to an equal volume of water. There may be two minerals that are the same size, but yet their weight varies. Water’s specific gravity is one. So if a mineral has a specific gravity of less than two, it is a very light mineral, like borax. If it is three, it is a light mineral, such as gypsum or halite. If it is between three and five it is average, like mica or barite, and if it is between five and ten, it is heavy, like galena and pyrite . Greater than 10 is very heavy, such as gold or silver. Greater than 20 is super heavy. This would be platinum. Generally, if a mineral has a metallic luster it is a heavy mineral.
Some of the specifics are better determined if a pocket field guide is employed.