In the case of minerals and gemstones, hardness refers first to scratch hardness, then to cutting resistance. Today the hardness test is only rarely applied to precious stones and then mainly by collectors.
Friedrich Mohs (1773-1839), a Viennese mineralogis, invented the scratch hardness test. It was simply described that scratch hardness is the resistance of a mineral when scratched with a pointed testing object. Ten minerals were selected for comparison and graded from one to ten.
|Simple hardness tester||Cutting hardness|
|1||Talc||Can be scratched with fingernail||0.03|
|2||Gypsum||Can be scratched with fingernail||1.25|
|3||Calcite||Can be scratched with copper coin||4.5|
|4||Flourite||Easily scrathed with knife||5.0|
|5||Apatite||Can be scratched with knife||6.5|
|6||Orthoclase||Can be scratched with steel file||37|
|7||Quartz||Scracthes window glass||120|
Many gems can be split along certain flat planes, calls cleavage. But some gems cannot be cleaved at all like quartz. But you don't have to worry about this. Mostly lapidaries (a person who cuts gems) and stone setters will take account of the cleavage.The largest diamond of gem quality ever found, the Cullinan (3,106 carats!), was cleaved in 1908 into three large pieces.
Specific Gravity (Relative Density)
Specific gravity is the weight of a specific material compared with the weight of the same volume of water. For example, a gem with specific gravity of 2.6 means it is 2.6 times heavier than water both with the same volume. The value varies between 1 and 7. Values under 2 are considered light (amber is 1.1); those between 2 and 4 considered normal (quartz is 2.6); and those above 4 are heavy (cassiterite is 7). Below is specific gravity of some common gems.
|Specific Gravity (Relative Density)|
|1||Ruby||3.97 - 4.08|
|2||Sapphire||3.99 - 4.00|
|3||Alexandrite||3.70 - 3.73|
|4||Rhodonite||3.40 - 3.70|
|5||Spinel||3.58 - 3.61|
|6||Topaz||3.53 - 3.56|
|7||Diamond||3.47 - 3.55|
|8||Peridot||3.27 - 3.36|
|9||Lapiz Lazuli||2.40 - 2.90|
|10||Turquoise||2.60 - 2.80|
|11||Emerald||2.67 - 2.78|
|12||Pearl||2.60 - 2.78|
|13||Aquamarine||2.67 - 2.71|
|14||Tiger's Eye||2.64 - 2.71|
|18||Amethyst||2.63 - 2.65|
|19||Agate||2.60 - 2.65|
|20||Moonstone||2.56 - 2.62|
|21||Opal||1.98 - 2.20|
|22||Amber||1.05 - 1.30|
Color is the most important characteristic of gems. In the case of most stones, it is not diagnostic in identification, because many have the same color and numerous stones occur in many colors. Color is produced by light; light is electromagnetic vibration at certain wave lengths.
In gemstones, the metals, mainly chrome, iron, cobalt, copper, manganese, nickel and vanadium, absorb certain wavelengths of white light and so cause coloration. The distance the light travels through the stone can also influence absorption and thus color. Gems cutter would know that light colored stones should be made thicker, while stones that are too dark are cut thinly.
Although color is very important in gems (with the exception of diamonds), no practical method is known. Color comparison charts are poor substitutes because there is so much room for subjective consideration.
The color of some gems is altered by time. Amethyst, rose quartz, and kunzite can become paler when exposed to direct sunlight. But in nature, color change by nature causes are not common.
Less attractive colors can be changed to more desirable hues by heating, often times accompanied by certain chemical. Best known is the heat treatment of ruby and sapphire. The resulting colors are sometimes so close to nature that they cannot be detected by naked eye. They have to be marked as color treated or heat treated when offered for sale.
Transparency is an important factor when evaluating gemstones. Inclusions (of foreign matter) and air bubbles in the inside of gemstones affect the transparency and value. The path of light through the gems can also be impaired by strong absorption of the gem itself.
The luster of a gem is caused by reflection. It is dependent on the refractive index and the nature of the surface, but not the color. The higher the refraction, the higher the luster. The most desirable luster is diamond, and stones with no luster are described as dull. The light effects which are caused by total reflection are considered as luster. The lower facets of the gem act as a mirror and reflect the entering light, so strengthening the lustrous appearance.