The second section to be explored focuses on mineral identification. Minerals have special important properties. It is necessary to know these properties in order to identify samples. Important properties include color, luster, hardness, cleavage and density. Looking at color alone is usually not sufficient to identify a mineral. A mineral should be scraped across the unglazed back of a tile to find it’s true color. The color of the powder mark is called the mineral’s streak color and the scraping procedure is called the streak test. Luster is also important in identifying minerals. Some minerals containing metals have a metallic luster or a shiny surface that reflects light. Other minerals can have glassy, silky or even greasy lusters. The hardness of a mineral is one of the most useful properties to identify. The hardness scale was invented by the mineralogist Friedreich Mohs in 1822. The scale is divided into 10 degrees of hardness and is based on the principle that any mineral can scratch another one of the same hardness or softer than itself. Diamond, the hardest natural mineral, has a hardness value of 10 and can scratch anything else. Minerals also split or break into pieces with flat, smooth surfaces. This splitting is called fracture or cleavage. Some minerals split very easily in certain directions by using a chisel and hammer. Many minerals will break easily only if they are hit in exactly the right place. Minerals and rocks may also break unevenly. Quartz doesn’t have a neat cleavage but instead fractures in various directions. Cleavage and fracture are important in identifying rocks and minerals. Another important way of identifying minerals is to use the density test. In order to find the density of a substance, it is necessary to measure both the mass and volume. By dividing the mass by the volume the density of a substance can be found. Specific gravity can be found by comparing the density of a substance to the density of water. Finding the specific gravity of an unknown mineral and comparing it to the constant specific gravity of known minerals is also very useful in mineral identification.
The concepts of density and specific gravity are particularly useful in mineral identification because density is a basic physical property of all matter. Every substance has a density that can be measured and the density of like substances is always the same. For example, the density of lead is always 11.3 grams (mass) per cubic centimeter (volume) and the density of iron is always 7.9 grams per cubic centimeter. Mass refers to the amount of matter in an object and volume refers to the amount of space something takes up. Density does not depend on the size or shape of the substance. For this reason density is extremely useful in identifying minerals by comparison.