Science Fair Project Encyclopedia Chalcedony is one of the
cryptocrystalline varieties of the mineral quartz, having a waxy
luster. It may be semitransparent or translucent and is usually
white to gray, grayish-blue or some shade of brown, sometimes
nearly black. Other shades have been given different names. A
clear red chalcedony is known as carnelian or sard; a green
variety colored by nickel oxide is called chrysoprase. Prase is
a dull green. Plasma is a bright to emerald-green chalcedony
that is sometimes found with small spots of jasper resembling
blood drops; it has been referred to as blood stone or
heliotrope. Chalcedony is one of the few minerals other than
quartz that is found in geodes. The contents of this article is
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