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How To Classify Gemstones
Precious and semi-precious gemstones:
A piece of mineral that is extremely striking and valuable is called a gemstone. Gems are, when cut and polished, used in jewelry. Some stones are too soft for jewelry, but are still called gems because of their color and beauty.
Gems are defined as precious or semi-precious. The distinction of precious and semi-precious dates back to the ancient Greeks, and is still used today. The distinction have changed slightly from culture to culture and over time, and today only four minerals are labeled as precious: Diamond, Ruby, Emerald and Sapphire.
(On the photo to your left, you see a Ruby in its natural state, before it is cut and polished into a gem).
This leaves all the other minerals in the semi-precious lot. The four precious minerals are all very translucent with fine, clear colors in their purest forms. They are also very hard, measured with the Mohs scale. (I'll explain what this is further down the page).
Gemstones are actually minerals and not rocks. A mineral is a natural substance, and is formed through a geological process. Their chemical composition are characteristic, they have a highly ordered anatomical structure and specific physical properties.
A rock on the other hand is a mass of minerals and mineraloids and does not have a specific chemical composition. However some rocks, like Lapis Lazuli are in the jewelry business still counted as a gem.
Today there are over 4000 known minerals, and there are over 130 species that are used as gems.
There are also some organic materials that are used as semi-precious stones, like Amber, Bone, Coral, Ivory, Pearl and Amolite. Today we know that they are not minerals, but since they have been used as gems long before we knew how to classify minerals, they are still referred to as gems.
Classification of minerals/gemstones:
A substance can not be classified as a mineral unless it has a solid substance and have a crystalline structure.
The classification can range from very complex to very simple, depending on the mineral.
In general minerals are classified by the following properties:
Hardness and toughness –The Mohs hardness scale
Hardness in a mineral is measured by the Mohs hardness scale. It was invented by a German mineralogist Friedrich Mohs. The scale is based on how easy one mineral can scratch another mineral, and is measured by 10 minerals:
Any mineral will be able to scratch a mineral that is softer and lower on the scale than itself. Diamond is the hardest mineral and Talc the softest.
Most semi-precious stones used for jewelry have a hardness of 5 and higher. Some softer ones are used, but scratch easily and must be treated with love and care.
However the scale is a relative scale, not proportional. For instance a mineral with hardness 8 will not be twice as hard as one with a hardness of 4, and Diamond is 40 x harder than Sapphire although it is only one step on the scale between them. Therefore some mineralogists claim that a hardness test should not be used to classify gemstones.
Regarless of that, it will tell you whether a gem is suitable for the type of jewelry you want to make or not.
Hardness and toughness are not the same thing. Hardness is how easily a mineral can be scratched. Toughness is how it resist breakage. For instance, a mineral like Diamond is very hard, but can break if it gets a blow. Jade which is a softer mineral, scratch easily, but is harder to break, leaving it as a much tougher mineral.
Color and streak:
Color may not be the safest way to classify minerals, as a lot of them look very much alike. Colors are due to impurities and deposits in the crystals, but also by how the light is absorbed in them.
Streak is the color a mineral get when pulverized. The drawback is that several minerals are white when powdered, so this is not so reliable either. Anyway the streak test can easily be done if you scrape the mineral against porcelain and the powder left behind show you the minerals color.
Some gems are Iridescence, which means that they have a play of colors, due to surface or internal interference. Hematite sometimes show the surface effect, while Labradorite shows the internal effect.
Fluorescence is another phenomena that some minerals exhibit. That means that their chemicals react with ultraviolet light, causing them to glow after the light has been removed. Thermoluminescence is when they glow when heated, and Triboluminescence is when they glow when struck or crushed.
The colors displayed with these three phenomena, can be very different from the normal colors of the mineral. Labroadorite, for instance, looks greyish until you see it from another angle. Then it will display fabulous sparkling colors of the deepest blue to fiery orange.
Luster in a mineral is how the light is reflected from it. This is one of the most reliable of the visual classifications. The way the light is reflected vary due to the surface of the gemstone. (Below on the right you can see the metallic luster of Actinolit). Luster is described with these terms:
Habit is the crystals appearance, shape and size of a mineral. It identifies its growth characteristics, which is unique and is a result from its crystalline structure and growth environment.
Normally minerals are not found as a single formulated crystal, but as many different crystals that have grown together into a mass. Their crystalline structure is sorted in the following groups:
Here you see how different the crystals in minerals are. You can also see that they grow into other minerals and their wonderful diversity in color and luster.
Cleavage in a mineral is how it breaks along a certain plane. This is dependent on the orientation of the minerals atomic bonding. A mineral can have from one to five cleavage plains. The plains are graduated by how easy the mineral can be cleaved. They are rated by:
The cleavage happens along a very smooth plane
Along a smooth plane
Is well defined
The surface have some imperfections
(Indistinct, Imperfect) Leaves a rough surface that is close to an irregular fracture.
The cleavage fractures along a random path, leaving a jagged surface
Diamond have for instance a perfect cleavage while Sapphire have none. The numbers of cleavage plains and the angle between them give each mineral a specific characteristic.
The cleavage is of importance when large gemstones shall be parted into two or several pieces. Although they today have saws to do the job, they still have to cut along the cleavage plain, or the stone may be prone to breakage.
The same goes when jewelry with gemstones set in them are soldered. The heat may cause the gem to break along the cleavage plane.
A fracture is when a mineral have broken in a direction other than the direction of the cleavage.
All minerals can fractured when put under stress, even if they have a perfect cleavage.
They tend to fracture in distinct ways, and can therefor be identified by how they fracture:
- Sub Conchoidal
- Step like
- Non observed
Fractures in gemstones are often filled with glass or resin to conceal them. You can read more about that on the Gemstone enhancement page
Twinning occurs when crystals of minerals grow together symmetrically. It occurs early in the crystal growth and happens when the growth is fast. Slow growth discourages twinning. Twins have different characteristic. They are also divided in two basic groups:
- Contact twins
- Penetration twins
Twins are durable and do not break apart easily. (Very much like human twins or maybe more like Siamese twins)
The definition of Density is the amount of mass per given volume. The density or specific gravity of a gemstone is, easily put, the number of times heavier a gemstone is than an equal volume of water.
Aquamarine for example has a specific gravity of 2.63 – 2.91, which means that a cubic inch of Aquamarine weigh 2.63 – 2.91 times more than a cubic inch of water. Gemstones have different density, meaning that two equally large gemstones weigh differently because of their density / Specific gravity.
A gems density/specific gravity is measured on a scale from 1 to nearly 7.
The crystalline properties of gemstones are intertwined. They all have a definite chemical composition though, and are divided into these groups:
The "Minerals" composed of organic chemicals
The "Minerals" that lack crystal structure
The Sulfides, Selenides,Tellurides,Arsenides, Antimonides,Bismuthinides and the Sulfosalts
The Sulfates, Sulfites, Chromates, Molybdates, Selenates, Selenites, Tellurates, Tellurites and the Tungstates
The Silicates (the largest class)
The Phosphates, Arsenates, Vandates and the antimonates