The varieties of crystal are divided according to their colors and the other features that they contain.
According to the colors, they can be divided into the following five categories:
(1) Crystal: Colorless and transparent crystal. In some countries, the crystal that is used as the gemstone often has the name of Diamond, such as Alaska Diamond, Arizona Diamond and so on.
(2) Amethyst: The purple and transparent crystalloid. It is of the more valuable members in the crystal family. It has been very popular with people for its beautiful purple and has the good reputation of king of crystal. It has apparent polychroism watching in the dichroscope. Under the conditions of high heat or burning, the color of the gemstone will be changed and become yellow brown, brown and so on.
(3) Topaz: The crystal which is from light yellow to deep yellow. It often has the yellow of yellow gemstone.
(4) Smoky Quartz: The crystal which is from smoky yellow, brown to dark. In some countries, smoky yellow and brown crystal is also called Tea Crystal; black crystal is called Ink Crystal. The color of the crystal was once thought to be caused by the impurities contained; according to the research nowadays, its color is caused by the tiny radioactive element (Radium) it contains. We use other physical methods, like X radiation, heat radiation etc., can change the color of smoky crystal into yellow.
(5) Green crystal: The green and transparent crystalloid.
According to other features, we can divide the crystal into the following five categories:
(1) Water Tank Crystal: The crystal contains visible large liquid inclusion. In Beijing Gemological Museum, there is a water tank crystal which is as big as a fist and it includes a liquid inclusion whose diameter is about 3.00cm.
(2) Hair Crystal: The crystal which contains needlelike mineral inclusion. It can also be divided into the following types:
① Acicular Crystal: The transparent crystal contains needlelike rutile inclusion. The rutile inclusion is arranged disorderly.
② Tourmaline hair crystal: The crystal contains needlelike tourmaline inclusion. The tourmaline inclusion is also arranged disorderly.
③ Actinolite Hair Crystal: The transparent crystal contains needlelike actinolite inclusion. The actinolite inclusion in the crystal is arranged radially.
④ Gold Hair Crystal: The transparent crystal contains gold-hair-like natural gold inclusion which is a kind of very rare and precious gemstone.
(3) Rainbow Crystal: It is the kind of crystal which contains tiny bubble or liquid and they fill the cracks. When the light goes through these cracks, they will cause interference and form the interference colors just like the rainbow. This kind of crack can be caused naturally or artificially (Heat up first and cool it down suddenly).
(4) Sparking Crystal: The crystal contains hematite and mica sheet inclusions. When the light shines these inclusions, the crystal takes on from red yellow to light yellow and sparking brightly.
(5) Shining Crystal: It is a kind of special crystal which has strong phosphorescence and it is often regarded as Crystal Shining Stone or Crystal Legendary Luminous Pearl.
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