Physical Properties of Gemstones

The specific gravity of a gem is its weight when compared with the same volume of water at a temperature of 4 degrees Celsius. The denser the minerals in the gemstone are, the heavier the weight or specific gravity will be. Heavier gemstones are usually harder as well. The range is from amber, which has a specific gravity of 1.08 and opal, with a specific gravity of 2.05, all the way up to corundum (sapphires and rubies) with a specific gravity of 3.99, spessartite garnet, specific gravity of 4.15, marcasite, specific gravity of 4.9, and cuprite (s.g., 6.0) and casseterite (s.g., 6.9). Diamond is in the heavy mid-range, with a specific gravity of 3.52.

Gemstones are often tested by using the Mohs' hardness scale to determine just how hard they are. The harder minerals are more durable in that they do not scratch easily and will hold up better in jewelry. Talc is the softest mineral with a rating of 1 and can be easily scratched with a fingernail. The gmestones with a rating of 7 or over are relatively hard. Quartz gemstones (citrine, amethyst, etc.) range in the 7's, topaz rates 8, and corundum (sapphires and rubies) are a 9 on the Mohs' hardness scale. diamond registeres a 10 and is the hardest known material on earth, more than ten times the hardness of corundum at 9. There is more of a spread between the gems and minerals found between 2 and 3 and between 5 and 6, however corundum is only about 10 per cent harder than topaz. the hardness is relative, but it is, nevertheless, a useful identification tool.

Cleavage is the splitting of gems and minerals along one of the planes related to the stone's structure. Crystalline minerals have cleavage and fracture, whereas amorphous or massive stones only fracture. Cleavage is considered perfect or eminent if the stone produces perfect smooth planes (diamond, topaz, gypsum, calcite) and is most important in diamond-cutting. It is good in flourite and pyroxenes, imperfect or poor in garnet, berly and zircon. massive quartz has no cleaveage. Fracture is the way a stone breaks. Conchoidal fracture shows a series of arcs that spread outward (quartz, opal, flint, obsidian). Even fractures occur in garnet, uneven fractures in pyrite, slintery or hackly fractures in serpentine, and earthy fractures occur in chalk and sepiolite.

Tenacity is the ability of a stone to withstand pressure or impact. Minerals which crumble into small pieces or a powder are said to be brittle. If a gem bends but returns to its original position, it is said to be elastic (mica, nephrite, jadeite); these minerals are tough and difficult to break. The jade gemstones (jadeite, nephrite) are the toughest of all gems, making them also difficult to cut. Talc and gypsum are examples of minerals which are flexible. Ductile or malleable minerals are those (gold, silver, etc.) which may be flattened out into thin sheets under pressure. The brittleness factor of a gemstone is an important consideration in gem cutting and polishing. Many gem crystals shatter or chip easily, and this must be taken into consideration when cutting.

Those stones which are attracted by a magnet are considered magnetic, such as magnetite and hematite, which contain iron. Most minerals and gems are poor conductors of electricity. Good natural conductors include native metals and minerals with a metallic lustre (pyrite). Natural blue diamond is a semi-conductor. Some stones, such as tourmaline, become electrically charged when heated and are said to be pyroelectric. Tourmaline is also piezoelectric; it becomes charged if stressed at certain points along the crystal. Quartz is an important piezoelectric minerals and this factor is what makes it useful in electronic circuits and photoelectric processes. Amber is triboelectric; it develops a negative electric charge when it is rubbed and attracts small fragments to its surface.

Finally, some stones are good conductors of heat, such as quartz, which draws heat away from the body when held and thus feels cold to the touch. A poor thermal conductor, such as amber, feels warm to the touch because it does not conduct heat away from the body. The surface of a genuine gemstone will de-mist more rapidly than that of glass or an artificial stone. Thermal conductivity should also be considered when cutting gemstones, as some stones will need a cooling-off period during the cutting.

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