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Some Basics of Quartz Crystals
May 15, 2005

CLEAR QUARTZ

Rock crystal is transparent and colorless quartz. It commonly occurs in quartz veins where it crystallizes inside rock cavities known as vugs. Most rock crystal forms on a matrix of milky quartz and crystals of rock crystal often grade into milky quartz towards their base

Phantom quartz usually occurs in rock crystal, but is also found in smoky quartz, citrine, and amethyst. Rock crystal is transparent and colorless quartz. It commonly occurs inside quartz veins where it crystallizes in rock cavities known as vugs or pockets Phantom quartz crystal shapes can sometimes be seen in the interior of quartz crystals, outlining an earlier stage of the crystal's formation. These phantoms are usually composed of other minerals such as chlorite, goethite or hematite or are composed of other varieties of quartz such as milky quartz, smoky quartz or even amethyst which form on most or all of the surfaces of the quartz crystal at a particular point in time during its growth, after which the quartz crystal resumes its crystallization enclosing the phantom crystal outline within itself. Phantom quartz is recognized by its characteristic phantom crystal within itself. It can be identified as quartz by its crystal habit, transparency, hardness, and glassy luster.

COLORED QUARTZ

Amethyst has been found in siliceous volcanics, occurring as macroscopic crystals and druzy coverings inside of agate lined cavities or vugs, often forming geodes. Amethyst also occurs in quartz veins. The purple color of amethyst is due to small amounts of iron impurities at specific sites in the crystal structure of quartz.

Citrine is less common in nature than amethyst, and it has the same general occurrence. Citrine has been found in siliceous volcanics, occurring as macroscopic crystals and druzy coverings inside of agate lined cavities or vugs, often forming geodes. Citrine also occurs in quartz veins. The color of citrine is due to small amounts of iron impurities in the crystal structure of quartz. The difference between citrine and amethyst is only the oxidation state of the iron impurities present in the quartz. Subjecting amethyst to heat will reduce the oxidation state of the iron impurities in the quartz structure thereby causing amethyst's purple color to fade and become yellow to reddish-orange (citrine), green, or colorless depending on the site and original oxidation state of the iron impurities present and depending on the amount and duration of the heat applied.

Ametrine is the name used for quartz in which both amethyst and citrine occur together. Iron atoms may enter the crystal structure of quartz as an impurity in a number of sites and in different oxidation states. For Ametrine to occur, iron impurities in the oxidation state must be present in the specific color producing sites in a portion of the quartz and must be present in the specific color producing sites in another portion of the quartz. There are a number of processes by which iron can be present in more than one oxidation state in different parts of the same specimen, and they are not all fully understood. Ametrine is natural but can be man made. The most well known natural Ametrine source is the Anahi mine in Bolivia.

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