Chalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy luster. Chalcedony 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 and onyx is black and white banded. Plasma is a bright to emerald-green chalcedony that is sometimes found with small yellow spots of jasper resembling, and Bloodstone is similar to Plasma but with red spots of jasper, thus the name Bloodstone, it is also called heliotrope. Flint is also a variety of chalcedony. People living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios. Hot wax would not stick to it so it was often used to make seal impressions. The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadiköy district of Istanbul.