|Semi precious gemstones|
Photo by Mauro Cateb
All gemstones are minerals, but not all minerals are gemstones, so the limited knowledge imparted here can be used on both gemstones and minerals. If you can’t tell what kind of gems you own don’t feel bad because there have been plenty of emperors and kings that didn’t have a clue about what kind of stones they had in their own crown. A good example of this is the Black Prince’s Ruby that resided in the Crown of England for hundreds of years before somebody found out it was really a mineral called “spinel.” This is a stone that looks like a ruby but is a much less expensive stone then real ruby. Clear white topaz is often mistaken for diamond and in the past many large diamonds have been found to be really topaz. Identifying these stones is in the realm of the gemologist who is specially trained to identify gemstones.
|A translucent ruby|
Photo by Rob Lavinsky
There are several stones that are treated as gemstones, but aren’t. Amber is a glaring example of this, a stone that is treated as a gem but in reality is fossilized tree sap having many inclusions of insects and other organic matter. Petrified wood is another example that is often beautiful and is treated by many as a gemstone, but isn’t. Jet, the black stone is actually a variety of coal and far from the definition of a gemstone, as are pearls. Pearls have the distinction of being the most valuable stone, but they aren’t gemstones either.
Identifying gemstones and minerals is really a process of the elimination, for instance if you are presented with a green clear gemstone it is not apt to a ruby. More than likely it is emerald, but you really can't be sure without further tests. There is a variety of garnet called demantoid that is also green and is often mistaken for an emerald. One of the most important tests that are used in mineral identification is the color of the stone.
Hardness is another criterion that is used. This is based on the Moh’s hardness scale that ranges from 1 to 10 with diamond being the hardest and soapstone the softest. Most stones that are used in making jewelry are harder than seven on the Moh’s scale even though some stones that are as soft as three on the Moh’s scale are faceted for collectors. You can buy hardness testing kits or make your own from various minerals that work’s as good as store bought.
|Turquoise from Mojave County, California|
Photo by Rob Lavinsky
The looks of the stone are important is it transparent, translucent or opaque. Transparent means can you see through the stone so things can be seen clearly that are behind the stone. Translucent means does light pass through the stone, but you can’t make out objects that are in back of the stone. Opaque means light doesn’t pass through the stone, and you can’t see through it.
Streak is literally a thin mark of the mineral or gemstone composed of very fine powder left on a piece of unglazed porcelain colored either black or white. Black is often used to test for gold and other precious metals when it is known as a “Touchstone.” It is often used in conjunction with acid to see if there is a reaction between the streak and the acid. The karat weight of gold can be determined by treating its streak with various concentrations of nitric acid.
|2x2 inch black and white streak plates, White is pyrite and black is rhodochrosite|
We sell streak plates: Click Here
Cleavage and fracture are other important tests. Fracture is most important for quartz gemstones that break in a Concoidal fracture that looks like little clam shells. Some minerals like jade have a hackly fracture i.e. a rough appearance. Some stones have a pronounced cleavage like topaz or diamond. Topaz is especially prone to a perfect basal . cleavage causing it to be a poor gemstone.
Specific Gravity was discovered by Archimedes when he was tasked to find out if a gold smith cheated the Tyrant of Syracuse when he was making a golden crown for the tyrant. This was a job that required quite a lot of forelock tugging on the part of Archimedes until he finally threw his hands up and decided to take a hot bath. His slave girl filled the bathtub to full of hot water so on Archimedes claimed into the tub it overflowed. He had discovered the principle of specific gravity, and with that he jumped out of the bathtub and went streaking through the town agora of
yelling at the passersby Eureka, Eureka
that means I have found it, I have found it. They all thought he had gone nuts,
and if it wasn't for the fact his brother-in-law was the tyrant he will
probably been slapped in jail.
What Archimedes discovered was the body would displace an equal amount of water according to its volume, or in modern terms its weight per cubic centimeter. This is normally taken by first weighing the object in air and then weighing in water. The difference is its Specific Gravity.
These are some simple tests that can help with gemstone or mineral identification there are others that are far more complicated that can be used in conjunction with the simple tests. Although there are over 2000 recognized minerals in the world there are only about 300 that are common rock forming minerals that you are apt to encounter in the field. A good source of information about most minerals can be found at www.mindat.org.