[Illustration: QUARTZ (obscurely crystalline.) LOANED BY FOOTE MINERAL CO.]
Left column: Bloodstone, polished (India.) Tiger Eye, polished (South Africa.) Center: Chrysoprase, polished (Arizona.) Agate and Carnelian, polished (Lake Superior.) Right column: Jasper (Germany.) Ribbon Jasper, polished (Siberia.)
Carnelian is a red variety of chalcedony, sard a brown variety. All gradations between these shades of course occur, those of the reddish cast being the most common. The most highly prized color for carnelian is a deep blood red, appearing darker red in reflected light. The lighter red and yellowish shades are less desirable, stones of these shades being known as "female carnelians," while those of the darker shades are known as "male" carnelians. The colors are due to oxides of iron and can sometimes be changed by heating. Thus the yellowish and brownish carnelians being colored by iron hydroxide can be changed by heating to red, the water being driven off and iron oxide left. The heating may be done in the sun or by some other slow means. Even olive green stones are changed in India to red by this process. The color may also be introduced artificially by allowing the stones to lie in a mixture of metallic iron and nitric acid, or of iron sulphate for a while. In this way the iron salt needed for the coloring matter can be absorbed by the stone and this is changed afterwards to oxide by heating. The best carnelians come from India, but good stones are also obtained in Siberia, Brazil and Queensland. Carnelians are cut usually in oval and shield-like shapes and were much employed by the ancients for intaglios. They believed them to have the power of preventing misfortune and they were much worn as charms.
Sard of typical brown color is much rarer than carnelian and possesses a high value. In other respects it is like carnelian. The sardius mentioned in the Bible as forming one of the stones of the High Priest's breastplate was undoubtedly a carnelian. The name was derived from Sardius, a city of Lydia whence fine carnelians are obtained. The name carnelian is according to some authorities derived from the Latin word caro, carnis, flesh, and refers to the color of the stone, or according to others it is from the Italian word carniola which has the same meaning.