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 Mechanical properties |
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 Density |
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 The density of a stone is the ratio of its weight to that of
its volume of water. A gem having a density of 2.8 thus weighs
2.8 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. |
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 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 5.8 grams and water 3.5
grams, the difference would be of (5.8 - 3.5
= 2.3 grams), that is to say a volume of 2.3
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 (5.8/2.3 =
2.5), this stone would thus have a density of 2.5. |
 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). |
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| Hardness |
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 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). |
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| Fragility |
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| 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 colour 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. |
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| 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. |
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| 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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0.68 carat D-VVS1 |
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1.53 carat H-VVS2 |
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0.26 carat E-SI2 |
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1.56 carat H-VS2 |
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0.44 carat H-VS2 |
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| Price: £ 1,910 |
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| Price: £ 6,767 |
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| Price: £ 181 |
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| Price: £ 7,926 |
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| Price: £ 526 |
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