The density of a stone is the ratio of its weight to that of its volume of water. A gem having a density of 2.80 thus weighs 2.80 times the weight of the same volume out of water.
The unit weight of the gems ranges between 1 and 7, those which are located below 2 (amber for example) are regarded as light, those ranging between 2 and 4 (for example quartz) are qualified as normal, and those above 4 are regarded as heavy.
Diamond has an average density of 3.52. This figure can vary according to its content of elements traces and inclusions. Thus the density of diamonds Australian can be 3.54, the density of certain yellow diamonds of Africa can be 3.524 and certain brown diamonds of Brazil can have a density of 3.60.
There are two methods to establish the unit weight, namely: method of weighing (which is practised using a hydrostatic balançe) and method of the Specific gravity. The first requires enough time and the second is rather expensive and can be employed for larger fragments.
Hydrostatic balance: the process rests on the Archimedes' principle, it' necessary to know the volume of the gem. The formula is as follows: one divides the weight of the stone by his volume.
For example, if we have a gemstone which weighs in the air 5.80 grams and water 3.50 grams, the difference would be of (5.80 - 3.50 = 2.30 grams), that is to say a volume of 2.30 cm3 water.
To find its density it would then be enough to divide its weight in the air by its volume, that is to say (5.80/2.30 = 2.50), this stone would thus have a density of 2.50.
Specific gravity: it's a very precise method some which is dimensions of the specimen. The principle remains very simple: a mineral runs in a liquid of which the gravity is less, but it will remain in suspension if the gravities are equal and will float if the liquid with a stronger gravity.
To determine the specific gravity of a stone, it will thus be necessary to find the specific gravity in which the gem will remain in suspension. There are several specific gravity liquid set in the trade, for example: the broforme (density = 2.90), liquid of Thoulet (3.19), the iodide of méthylème (3.33), the solution of Clerici (5).
Hardness can be defined by a mineral's resistance to being scratched. This process is hardly employed today by mineral collectors because it's not precise enough in gemmology. To measure this hardness we use the Mohs scale which comprises 10 values, from 1 to 10. The stones 1 and 2 are soft, those between 3 and 6 have an average hardness, those beyond 6 are known as hard. The test for hardness is carried out using sharp instruments of various hardnesses applied to le lowest part of the gem and starting with the lowest hardness first, until is scratche is made.
Diamond has a hardness of 10 (Mohs Scale).
The mechanical resistance impact is, in general, the opposite to the hardness of the stone. In other words, a stone of high hardness is more fragile to shocks. A diamond will be thus relatively fragile to shocks. Diamond has also a very strong elasticity which makes it rebound as a ball when it strikes a hard surface. A shock which could be insufficient to break a gem can cause dammages if applied repetively. A rock crystal hitting a diamond could often have less dammage than the diamond itself. The heat treatments used to improve the color of the gems weaken them by modifying their internal structure. A gem having a plan of perfect cleavage will be able to break easily according to this direction after the shock of a blade. For example, for diamond, we use this method to separate two pieces of rough diamonds, this operation is called: cleavage.
The thermal resistance impact is fatal for all gemstones. A variation of a hundred degrees (+/-) is a threat to all gemstones. Theoretically people will not have to undergo such thermal variations during their life, therefore the risk for the gemstone that one would carry is negligible. Be aware, the jewel is given for repair it can undergo the flame of a blowtorch or any source of heat which could have dammaging effect. On long term the stone could even split by itself.
The chemical resistance impact. A gem, must by definition resistance to normal houshold chemicals. Certain organic gems, such as the pearl, turquoise, rhodochrosite or the malachite can be sensitive to the fatty acids contents in the beauty creams and the perfumes. The stronger acids even manage to corrode peridot or the lapis lazuli. Concerning diamond there is no particular problems regarding these usual chemicals.
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