jasper

Opaque, fine-grained or dense variety of the silica mineral chert that exhibits various colours, but chiefly brick red to brownish red. Long used for jewelry and ornamentation, it has a dull lustre but takes a fine polish; its physical properties are those of quartz. Jasper is common and widely distributed, occurring in the Ural Mountains, North Africa, Sicily, Germany, and elsewhere. For thousands of years, black jasper was used to test gold-silver alloys for their gold content. Rubbing the alloys on the stone, called a touchstone, produces a streak the colour of which determines the gold content within 1 part in 100.