WoW! Junior Gemologist
Gem Species and Varieties
by Donald Clark CSM
What makes a Species?
... A few gems, like amber and pearls, have their origins in the vegetable kingdom. However, most gems are minerals.
... Minerals are defined by two factors; their chemical make up and their crystal habit. For example, the chemical make up of beryl is Be3Al2Si6O18. That means there are 3 beryllium atoms, 2 aluminum, 6 silicon, and 18 oxygen atoms in each molecule of beryl. Now, combine the chemistry with a hexagonal crystal structure and you have the definition of beryl.
... These two elements, chemical make up and crystal habit, have to be taken together. Why? The best example is to compare diamond and graphite. Diamonds are the hardest substance on earth. Graphite is the stuff pencil leads are made from. It is real soft.
... Both graphite and diamond have the same chemistry. They are composed of just one kind of atom, carbon. You have to consider how the atoms align with each other to show how they differ.
What makes a Variety?
... Most of our gem minerals are colorless in their pure state. However, few are found completely pure. Trace minerals find their way into the crystals as they form. These trace minerals lend color to the crystals.
... A good example is corundum. It is colorless in its pure state, but add a bit of iron and titanium and you have a blue sapphire. If, instead, you add a bit of chromium you have a red ruby!
... Corundum is a mineral species. Rubies and sapphires are varieties of corundum. They have the same chemistry and crystal structure as all other corundums. It is useful to distinguish these as varieties, because rubies and blue sapphires are special.
... Another good example is the beryl family. Pure beryl is called goshenite. It is colorless and not very interesting. However, if you add a bit of iron, it turns blue and becomes aquamarine. Now it is valuable and highly sought after. If, instead of iron, you added a bit of chromium to the beryl, it becomes an emerald. That is even more valuable!
... It is not just color that gives us varieties. Sometimes a special feature will also create a variety. Chatoyancy, (the sheen of a cat's eye gem,) is a good example.
... The term "cat's eye" by itself refers to the chatoyant variety of chrysoberyl. Other minerals need their name added. We have cat's eye moonstone, cat's eye tourmaline, etc. These are varieties of moonstone and tourmaline.
... Star stones are much like cat's eyes, except they have 4 or 6 rays of light flashing across their surface. The most common ones are star sapphire and star garnet. Star stones are special varieties of their species.
... A few gems exhibit a color change phenomena. That is where the color of the gem changes when you go from daylight to artificial light. The best know color change gem is alexandrite. It is a variety of chrysoberyl. There are also color change garnets, sapphires, tourmalines, and more.
... So, a species is a type of mineral. A variety is a mineral with special coloring or features.
Junior Gemologist Questions
1 Which two qualities define a mineral?
2 What makes a variety?
3 Name three special features might you find in a gemstone?
Junior Gemologist Answers
1 Which two qualities define a mineral?
... The answer is chemical make up and crystal habit.
2 What makes a variety?
...A variety is a mineral with special coloring, like an emerald being a variety of beryl, or special features.
3 Name three special features might you find in a gemstone?
...A gem might show a cat's eye or a star. Those are two special features.
...The rarest and most valueable feature is a color change.
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