Specific Gravity or Density
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 hardness of 1 and can be easily scratched with a fingernail. The gemstones 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 registers a 10 and is the hardest known naturally occurring 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. Hardness is almost never used as a separation test with gemstones since it is considered a destructive test and other nondestructive tests exist to enable separation and identification.
Cleavage and Fracture
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 if the stone parts and produces perfect smooth planes (diamond, topaz) and is very important in diamond-cutting.
Fracture is the way a stone breaks. Consider fracture to be similar to a piece of wood breaking in a direction other than the direction of it's grain. Conchoidal fracture, which is most common in gemstones, shows a series of arcs that spread outward.
When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture.
Tenacity or Toughness
Tenacity or toughness 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.
Magnetism and Electricity
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 luster (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 mineral 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.
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.
Although traditionally crystal systems are not part of the physical properties table of gemstones, but most mineral gemstones are crystaline, with thier atoms arranged in regular and symmetrical patterns. Crystal systems are classifed into seven different systems, according to the "minimum symmetry" of their faces.
This systems share the same axis of symmetry
The monoclinic system has a minimum symmetry of the one two-fold axis.
- Cubic or Isometric
Crystals in the cubic system have the highest symmetry. The minumum symmetry is four-fold axes.
This system is defined by one four-fold axis.
The minimum symmetry of this system is three two-fold axis.
Crystals of this system have no axis of symmetry, so gemstones within this system are the least symmetrical.
Important physical properties of gems found at Multicolour.com
The information provided below, when available, for your reference only and relates only to the gems sells at multicolour.com.