Gemology is the science of identifying gems. It combines chemistry, physics (and in particular optics), as well as crystallography to determine a material.
The gems are mostly oxides, sulphates, aluminas, silica and silicates. White diamond is made of pure carbon. Most gems are colorless when their chemical composition is pure. These are usually impurities (ions) which react with the crystal structure to give the gem its color. For example, a pure corundum is colorless and it is then called leucosapphire. When it contains chromium impurities, it turns red and is called ruby. The same corundum with iron and / or titanium impurities will give a blue color: sapphire. There are almost all the colors of corundum, which we will then call sapphire, followed by its color: yellow sapphire or pink sapphire for example. All of these colors are caused by impurities. In laboratory,it is possible to analyze the chemical composition of the gem and its impurities. This makes it possible to determine what material it is.
Gems are also analyzed by their physical properties. There are three of them: their hardness, their density and their optical characteristics.
Density is measured by weighing the stone in the open air, then submerged in water. The ratio “weight in air” / “weight in water” will give us a value which is the density of the material. You can also assess the density of a stone by weighing it with your fingertips. Some gems have a density that is either very low or very high, which makes it possible to arouse avenues of research during gemological analysis. This technique only works with an unmounted stone. For a jewel, it will therefore be necessary to focus on other techniques.
Hardness is a characteristic of a material. Diamond has a hardness of 10, which makes it the hardest material on Earth. Nothing can scratch it! On the other hand, hardness does not mean indestructible. The diamond has cleavage planes which mean that in the event of an impact at a certain angle, it can be cut in two! The hardness test assumes that any material can only be scratched by material with higher hardness. We use pencils having a tip with a hardness of 1 to 10. We start from the less hard (1) to go to the hardest (10). We stop as soon as the pencil leaves a scratch. The hardness of the material is therefore between that of the pencil that scratched the stone and the hardness of the pencil just below. Personally, I never had the courage to scratch one of my stones!
Optics is the field of physics most used by the amateur gemologist. In fact, almost all of the instruments within reach of the amateur make it possible to identify a gem in relation to its optical characteristics: magnifying glass, dischrocope, polariscope, microscope, refractometer, etc. The use of these instruments is detailed in the Instruments section of the site.
Crystallography is also an element in the identification of gems. Indeed, we can buy rough stones and in this case, it is the shape of the crystal and its inclusions (if there are any) that will identify the crystal. Knowing the basics of crystallography also allows you to almost certainly identify the roughs that have been retouched. For more information, an entire chapter is devoted to crystallography here .
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