A gem of contrasts, onyx is a chalcedonic quartz frequently layered in different shades of contrasting color, most commonly the polar opposites of black and white. Found in both the Old and New Worlds, onyx was used by the Greeks and Romans to carve cameos and other personal jewelry; and by the tribes of ancient Mexico to make masks and sacred ritual objects. Alexander the Great particularly favored onyx, and many small cameo portraits of him survive from antiquity, when they were thought to bring the good fortune of this fabled ruler to the wearer. The gem is mentioned in the Old Testament book of Exodus as figuring prominently in the breastplate of the High Priest. Its lustrous, rich polished surface--usually of deepest ebony black or very pure white--makes it highly desirable stone for modern jewelry. The chief virtue of onyx as an amulet is that it seems to attract and absorb negative humors from the body of the wearer, causing relief from both physical pain and mental anxiety. Black onyx is especially thought to have unique properties, as black suggests both the presence of all colors and the absence of light. For that reason, it serves as a buffer from pain, a defensive wall against evil and illness.