Gemstones and Their Definitions

First of all, what are gemstones?

"Gemstone is a collective term for all objects used with ornamental stones for personal adornment that possesses beauty, durability, and stability. Beauty, a quality that varies among individuals, can be based on one or any combination of: color or lack of color, luster, transparency, or enhanced optical properties due to cutting and fashioning by the lapidary. Durability is the resistance of the gemstone damage and dependent upon physical properties, such as hardness and tenacity. Tenacity is the resistance to bending and breaking, while hardness measures the resistance to scratch. Diamond cannot be scratched by any other mineral and therefore, it is a gem with high hardness; diamond has low tenacity, in part due to perfect cleavage. Nephrite jade is relatively soft for a gem, easily scratched by dust in the air, but it is composed of fibrous crystals that resist breaking and therefore, it is tough or has high tenacity. Stability refers to the gem's ability to retain color inspite of heat, light, or chemical assaults.

Of the 4,000+ known minerals, 70 possess qualifications for gemstones, but of these, only approximately 20 are commonly encountered (Hurlbut & Kammerling, 1991, p. 3). These minerals include:

diamond,
corundum (ruby, sapphire, star ruby and sapphire),
beryl (emerald, aquamarine, morganite, goshenite, golden or heliodor),
chrysoberyl (cat's eye & alexandrite),
spinel,
topaz,
zircon,
tourmaline (indicolite, rubellite, schorl, elbaite),
garnet group (almandite/almandine, rhodolite, pyrope, grossular/tsavorite, spessartine, uvarovite),
quartz: crystalline (rock crystal, amethyst, citrine, cairngorm or smoky, rose, aventurine, tiger's-eye, rutilated),
quartz: cryptocrystalline (chrysoprase, carnelian, sard, bloodstone, agate, onyx, jasper, agatized or petrified wood),
olivine peridot (chrysolite),
jadeite jade,
tremolite-actinolite or nephrite jade,
spodumene (kunzite, hiddenite),
feldspar group (microcline amazonite, labradorite, orthoclase moonstone, oligoclase sunstone),
turquoise,
lapis-lazuli,
and opal."
(source: http://www.emporia.edu/earthsci/amber/go340/define.htm )

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Agate
This is a semi-pellucid variety of quartz. The colours are arraigned in strips or bands or blended in clouds and classified accordingly as moss, ribbon, dendrite, etc. The stone, widely distributed but not very popular, is generally brown. When used for ornamental purposes it has usually been dyed artificially as the porous nature of this mineral allows it to absorb material from solution.

Brownish- orange agate is known as cornelian, the green variety chrysoprase, the blue variety chalcedony and the brownish-red variety carnelian. Black agate is onyx.
(source: http://www.mineralszone.com/gemstones/agate.html)

Amazonite
Amazonite, also known as 'Amazone stone,' is a light, aqua-green stone with white mottled flecks. The name, Amazonite, has been taken from the Amazon River, where formerly certain green stones were obtained, but it is found in Brazil. The origin of the stone is the New England States and Colorado. The composition is basically of potassium feldspar, and it is a green variety of microline.

Amazonite is believed to be a sacred stone of high value. It was extensively used by the ancient Egyptians. It gives a cooling and soothing effect to the mental state. It is used for healing and spiritual growth. It is applied to align the heart and solar plexus chakras and also the etheric and mental bodies. The body energy currents are strengthened by the use of amazonite.
(source: http://www.mineralszone.com/gemstones/amazonite.html)

Amethyst
This is a variety of quartz coloured by traces of manganese, titanium and iron. It is of a pale lilac, mauve or bluish violet colour. Very popular in the past, its current value is stable as new mines have recently been discovered. Moreover, its worth has been further undermined over recent years by the flood of tasteless objects made in Brazil.

Amethyst has a very wide provenance, but the finest coloured specimens come from Brazil and the Urals. It is also mined in Madagascar, Ceylon, India and Australia.
(source: http://www.mineralszone.com/gemstones/amethyst.html)

Brecciated Mookaite
This is another new find from the outback of Western Australia. It occurs near the Kennedy Ranges about 100 miles inland from the coastal town of Carnarvon. This geologically interesting rock is actually a brecciated  radiolarite. The red cementing matrix does not undercut & takes a glassy polish.
(source: http://outbackmining.com/brecciated_mookaite.htm)

Citrine
A glassy, wine-yellow variety of quartz, often mistakenly called "topaz", which is a completely different and far more valuable gemstone.

The colour of citrine varies from pale yellow to Madeira. However, the darker citrine is generally artificially formed by heating poor quality amethyst.
(source: http://www.mineralszone.com/gemstones/citrine.html)

Druzy Quartz
Druzy Quartz (also spelt as Drusy or Druse) is a thin layer of quartz crystals covering the surface of a host stone. One good example of Druzy is the crystals filled inside the cavity of a geode. It is well known that amethyst or citrine get the source of their color from this quartz crystal but usually Druzy Quartz takes the host stone's color which is seen through it as in chrysocolla or uvarovite garnet.
(source: http://www.mineralszone.com/gemstones/druzy-quartz.html)

Garnet
The name given to a group of isomorphous minerals of different composition and colour.

Almandine is of a deep velvety red and the most widely used in jewelry making. In Austria and Czechoslovakia it is faceted and used for highly elaborate jewelry. In India it is cut as beads or en cabochon. The best stones are those that are not too dark.
(source: http://www.mineralszone.com/gemstones/garnet.html)

Lapis Lazuli
The "sapphire" of classical times, it has always been a popular stone on account of its fine blue colour. It is really a rock consisting of varying quantities of a blue mineral hauyne and calcite. Small specks and strings of the yellow iron pyrites are common.

The stone is becoming increasingly rare and its price rises steadily. It now costs more than gold and is the only opaque stone, apart from turquoise, sold by the carat (provided the quality is good). 

Golden threads in no way diminish the value of the stone and in fact are the means by which it may be identified. Lapis lazuli may be confused with sodalite, which has no golden threads, or blue aventurine, which has a schiller.
(source: http://www.mineralszone.com/gemstones/lapis-lazuli.html)

Labradorite
Labradorite, carries unique characteristic, turns sea blue, gold and green in bright light from grey or dark green color if viewed in dim light. It is a variety of plagioclase feldspar which is found in igneous rocks.

Polished labradorite is called labradorescence. The crystal is transparent to translucent. The darker variety is known as "black moonstone" which has a bluish inclusions.
(source: http://www.mineralszone.com/gemstones/labradorite.html)

Malachite
Malachite often results from weathering of copper ores and is often found together with azurite (Cu₃(CO₃)₂(OH)₂), goethite, and calcite. Except for its vibrant green color, the properties of malachite are similar to those of azurite and aggregates of the two minerals occur frequently together. Malachite is more common than azurite and is typically associated with copper deposits around limestones, the source of the carbonate.

In Israel, malachite is extensively mined at Timna valley, often called King Solomon's Mines, although research has revealed an interruption in mining activity at the site during the 10th century BC, the time of the biblical Solomon.^[3] Archeological evidence indicates that the mineral has been mined and smelted at the site for over 3,000 years. Most of Timna's current production is also smelted, but the finest pieces are worked into silver jewelry. In Greek mythology, the throne of Demeter, goddess of grain and harvest, was fashioned from malachite and adorned with golden pigs and ears of barley.[4]
(source: http://en.wikipedia.org/wiki/Malachite)

Mookite or Mookaite
Mookaite is the common, or popular, name for the rock with the geological name Windalia Radiolarite, being a fine grained, silicified and multi-coloured, radiolarian siltstone, found in outcrops, principally on Mooka Station (a sheep farm of around 700,000 acres)on the west side of Kennedy Range in Western Australia.
(source: http://www.mookaite.com/)

Obsidian
Obsidian, also known as Apache tears, is a volcanic glass which is generally black, but is occasionally red, brown, gray, green (the rarest variety), dark with "snowflakes," or even very clear. It is said, probably to be the most challenging stone in crystal work. It is in fact a volcanic glass and has been used in past by many native cultures to make knives.

There is also Snowflake Obsidian, which has small, white and radially clustered crystals of cristobalite in the black glass and it produces a blotchy or snowflake pattern in it.
(source: http://www.mineralszone.com/gemstones/obsidian.html)

Tiger's Eye
Tiger's Eye, much like the eyes of a female cat, glitters with a small ray of light on the surface. This property of stone is known as chatoyancy. It is a brown quartz silicon dioxide having lustrous yellow and brown parallel fibers. The presence of iron oxides gives the stone the color of yellow-gold.
(source: http://www.mineralszone.com/gemstones/tigers-eye.html)