|Quartz (var. ametrine)||NMNH G9950-00||Anahi Mine, Santa Cruz, Bolivia||24.15 ct|
|Quartz (var. ametrine)||NMNH G9576-00||Anahi Mine, Santa Cruz, Bolivia||55.68 ct|
Gift of John W. May.
Quartz is one of the most abundant minerals in the Earth’s crust. It is composed of the elements silicon and oxygen, and in its pure state is colorless. However, small amounts of various impurity atoms can yield a wide range of colors. Amethyst, the most prized gem variety of quartz, is purple. Citrine is the golden-yellow to orange variety of quartz. The gems pictured here are part amethyst and part citrine, known in the trade as ametrine. Even though ametrine is a naturally occurring variety of quartz, most ametrine is artificially created by heat treatment and/or radiation of amethyst. The color zoning in ametrine is due to differing oxidation states of iron inside the crystal. Most ametrine is mined in Bolivia, however there are deposits in Brazil and India. The pear-shaped ametrine weighs 55.68 carats, while the rhomboid shaped step-cut ametrine weighs 24.15 carats. Both are from Bolivia and currently on display at the National Museum of Natural History.
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