Black Agate (Onyx) which is truely a died black Agate, is more common and perhaps the most famous variety, but not as common as natural Onyx. Onyx is a crypyocrystalline form of Quartz. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). A picture of a true Black Onyx specimum is seen below. True specimens of Onyx contain bands of colors of white, tan, and brown. As stated, the pure black form which most people know as Onyx, is not a naturally occuring variety. Black Agate or poorly colored Onyx is heated and dyed black to come up with the pure black form so well liked within the jewelry industry.
Striped Onyx is a cryptocrystalline form of Quartz. Striped Onyx is a redundant statement since true Onyx will have color bands that range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of Onyx that are seen in the market place contain bands of colors of white, tan, and brown. Pure black Onyx is actually Dyed Agate or poor grades of Onyx where the stripes, as seen below, have been dyed to black.
Agate is a microcrystalline variety of quartz, chiefly Chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks. White Agate or probably better known as Greek Agate, is a name given to pale white to tan colored Agate originally found in Sicily back to 400 B.C. The Greeks used it for making jewelry and beads. Today almost any agate of this white color is called Greek agate. Yet the stone had been around centuries before that and was known to both the Sumerians and the Egyptians, who used the gem for decoration and religious ceremony. True White Greek Agate comes only from Sicily, once a Greek Colony.