Colored
Stones: Physical Properties of Gemstones
Birthstones
| Varieties | Properties
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.
| Diamond |
10 |
| Silicon carbide |
9 1/4 |
| Corundum &
Syn. |
9 |
| Chrysoberyl |
8 1/2 |
| Syn. cubic zirconia |
8 1/2 |
| YAG |
8 1/4 |
| Spinel &
Syn. |
8 |
| Painite |
8 |
| Topaz |
8 |
| Taaffeite |
8 |
| Rhodizite |
8 |
| Beryl &
syn. emerald |
7 1/2-8 |
| Phenakite |
7 1/2-8 |
| Gahnite |
7 1/2-8 |
| Gahnospinel |
7 1/2-8 |
| Zircon (high/medium) |
7 1/2 |
| Almandite garnet |
7 1/2 |
| Hambergite |
7 1/2 |
| Euclase |
7 1/2 |
| Rhodolite garnet |
7-7 1/2 |
| Pyrope garnet |
7-7 1/2 |
| Spessartite garnet |
7-7 1/2 |
| Tourmaline |
7-7 1/2 |
| Andalusite |
7-7 1/2 |
| Iolite |
7-7 1/2 |
| Staurolite |
7-7 1/2 |
| Grossularite garnet |
7 |
| Quartz &
Syn. |
7 |
| Danburite |
7 |
| Dumortierite |
7 |
| Chalcedony |
6 1/2-7 |
| Peridot |
6 1/2-7 |
| Jadeite |
6 1/2-7 |
| Andradite garnet |
6 1/2-7 |
| Axinite |
6 1/2-7 |
| Saussurite |
6 1/2-7 |
| Idocrase |
6 1/2 |
| Scapolite |
6 1/2 |
| Kornerupine |
6 1/2 |
| Zircon (low) |
6 1/2 |
| |
| Pollucite |
6 1/2 |
| GGG |
6 1/2 |
| Spodumene |
6-7 |
| Sinhalite |
6-7 |
| Epidote |
6-7 |
| Sillimanite |
6-7 |
| Cassiterite |
6-7 |
| Zoisite |
6-7 |
| Rutile &
Syn. |
6-6 1/2 |
| Microcline |
6-6 1/2 |
| Albite-Oligoclase |
6-6 1/2 |
| Orthoclase |
6-6 1/2 |
| Nephrite |
6-61/2 |
| Pyrite |
6-6 1/2 |
| Benitoite |
6-6 1/2 |
| Marcasite |
6-6 1/2 |
| Prehnite |
6-6 1/2 |
| Ekanite |
6-6 1/2 |
| Amblygonite |
6 |
| Labradorite |
6 |
| Leucite |
6 |
| Petalite |
6 |
| Hematite |
5 1/2-6 1/2 |
| Rhodonite |
5 1/2-6 1/2 |
| Beryllonite |
5 1/2-6 |
| Anatase |
5 1/2-6 |
| Brazilianite |
5 1/2 |
| Enstatite |
5 1/2 |
| Willemite |
5 1/2 |
| Moldavite |
5 1/2 |
| Thomsonite |
5 1/2 |
| Opal |
5 1/2 |
| Diopside |
5-6 |
| Glass |
5-6 |
| Strontium titanate |
5-6 |
| Lazulite |
5-6 |
| Lazurite (lapis lazuli) |
5-6 |
| Turquoise |
5-6 |
| Sodalite |
5-6 |
| Chlorastrolite |
5-6 |
| |
| Sphene |
5-5 1/2 |
| Obsidian |
5-5 1/2 |
| Datolite |
5-5 1/2 |
| Bowenite (serpentine) |
5-5 1/2 |
| Apatite |
5 |
| Scheelite |
5 |
| Dioptase |
5 |
| Smithsonite |
5 |
| Odontolite |
5 |
| Stibiotantalite |
5 |
| Syn. turquoise |
5 |
| Apophyllite |
4 1/2-5 |
| Syn. Opal |
4 1/2 |
| Zincite |
4 1/2 |
| Kyanite |
4-7 |
| Variscite |
4-5 |
| Augelite |
4 |
| Fluorite |
4 |
| Rhodochrosite |
3 1/2-4 1/2 |
| Malachite |
3 1/2-4 |
| Azurite |
3 1/2-4 |
| Sphalerite |
3 1/2-4 |
| Coral |
3 1/2-4 |
| Conch pearl |
3 1/2 |
| Calcite |
3 |
| Verdite |
3 |
| Black coral |
3 |
| Hemetine |
2 1/2-6 |
| Pearl |
2 1/2-4 1/2 |
| Jet |
2 1/2-4 |
| Pseudophite |
2 1/2 |
| Agalmatolite |
2 1/2 |
| Serpentine |
2-4 |
| Amber |
2-2 1/2 |
| Copal |
2 |
| Alabaster |
2 |
| Stichtite |
1 1/2-2 |
| Steatite (soapstone) |
1-2 1/2 |
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).
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