Gemology is the science and profession of identifying and evaluating gemstones. It is a science in the geology field and a branch of mineralogy. Some gemologists specialize in specific gemstones such as diamonds or emeralds while others are laboratory gemologists and identify a stones origin. Recently, because of an increase in the quality of synthetic gemstones, the demand for highly qualified gemologists has increased to identify and classify natural and synthetic stones. Gemologists study rough state of gemstones and the host rock rock as well as 5 properties of minerals and gems.
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Color is when light passes through a mineral and it is absorbed, refracted, reflected or passed through.
Gemologists study the color of the stone to determine true color or whether the stone color has been change by impurities. Stones with the same chemical composition can appear indifferent colors due to these impurities. For this reason, color is not relied upon as the only method of identification.
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Hardness is the stone's ability to resist scratches and breakdown. Hardness is measured on the Mohs scale and ranges from 1 to 10 with 1 being the softest and 10 being the hardest. Talc is at number 1 on the Mohs scale and it is easily scratched by a fingernail. Diamonds are at 10 on the scale with gemstones like topaz falling as an 8 and quartz at a 7.
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Lustre is the appearance of the stone or mineral, basically how shiny the stone is. In general, the higher the stone is on the Mohs scale, the more lustre or shine it produces. Thus, talc has no lustre while diamonds are very shiny and reflect light well. Some lustre terms are pearly like nacre, resinous like amber, adamantine like a diamond, earthy like marble, silky like serpentine, greasy like moss agate or vitreous like spodumene.
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Cleavage is how a mineral splits. A mineral has cleavage if it splits along a surface related to its mineral structure and doesn't shatter. There are 4 different types of cleavage: perfect, distinct, indistinct and nonexistent, ranging from best cleavage to no cleavage.
Twinning is when two crystals appear to be growing together as in twins. An example of a twinning mineral is pyrite.
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Chemical analysis is a final resort for a gemologist because analyzing a mineral in this way usually damages or destroys the sample. Chemical analysis involves three tests. Gemologists test solubility in acid where a mineral's ability to withstand acid can identify its properties. For example, copper dissolves in nitric acid only and gold is insoluble in any acid. Gemologists test for gases that are produced when samples are chemically altered such as sphalerite giving off a sulphur smell in hydrochloric acid. Placing minerals directly in flame shows gemologists the temperature at which they melt, if at all. Also, stones may give off different flame colors according to mineral content.
