Colored Stones:  Physical Properties of Gemstones

Specific Gravity 

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.

Hardness 

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.

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.

Thermal Conductivity 

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.

Optical Properties of Gems

Luminescence 

Luminescence incorporates a gemstone's ability to emit visible light in darkness when exposed to ultraviolet light (fluorescence, named after flourite, the predominant flourescent gemstone), and in the case of kunzite, to produce an "afterglow" which lingers after the light has ceased (phosphorescence).

Luster 

The luster or brilliance of transparent gems is caused by light reflecting from the stone's surface. The smoother and more highly polished the surface is, the greater the luster will be. High light refractivity of a gem will cause greater luster as well. The most intensive luster is seen in the highest refractive indices, diamond, zircon, and rutile, and is known as an adamantine luster. Hematite produces a metallic luster, even though it is not transparent. Most gemstones have a vitreous or glassy luster, but there are other types of lusters, including resinous (amber), greasy (serpentine), waxy (turquoise), pearly (rhodonite), and silky (tiger's eye).

Refraction and Birefringence 

Refraction is the bouncing around of light from the greater part of the light ray which hits the gemstone and passes into the stone. As it enters the denser medium of the gem, the light bends and the amount of bending or light refraction produces a measurable index (refractive index), which is often used to help identify a gemstone. When light hitting a gemstone splits into two rays traveling through the stone at different speeds and in different directions, the reaction is called birefringence or double refraction. This is seen uncommonly and in a variety of calcite called Iceland Spar as well as zircon, rutile, and sphene.

Dispersion 

Dispersion is the separation of light into its separate spectral colors. Gemstones with the highest light refraction typically show the highest dispersion rate as well (rutile, sphene, diamond, zircon). This color dispersion or fire can be enhanced by a gem cutter if he uses an appropriate facetting style.

Pleochroism 

Color changes which are evident when viewed from different angles in gemstones (iolite, alexandrite, andalusite) is called pleochroism. It is very important for the gem cutter to cut a pleochroic stone properly in order to show off the different colors. Ruby and sapphire have two color shades and are pleochroic; in ruby, for example, yellow-red and purplish-red, which distinguishes it from garnet and red spinel, which have no pleochroicism. Iolite displays lavender-blue, gray, and pale yellow when viewed from different angles. 

Luminescence

Luminescence incorporates a gemstone's ability to emit visible light in darkness when exposed to ultraviolet light (fluorescence, named after fluorite, the predominant fluorescent gemstone), and in the case of kunzite, to produce an "afterglow" which lingers after the light has ceased (phosphorescence).