Garnet and Spinel CS Book 5 AS-24 PDF

Garnet and Spinel KEY CONCEPTS Garnet and spinel are rarely treated. Hessonite is the transparent orange to cinnamon-colored...

Garnet and Spinel KEY CONCEPTS Garnet and spinel are rarely treated. Hessonite is the transparent orange to cinnamon-colored variety of the grossular species. Garnet and spinel are singly refractive, eliminating the need for cutters to consider pleochroism. Most red garnets, including the popular rhodolite, are mixtures of pyrope and almandine. Garnets belong to a group of minerals with the same crystal structure but slightly different chemical composition. Rhodolite—the most valuable red garnet—has a dark purplish red to light reddish purple hue. Many garnets are mixtures of two or more garnet species. Gemologists classify garnets by their physical and optical East Africa has been the most important source of rhodolite properties, but many dealers sell garnets on the basis of color since the 1960s. or origin. Huge numbers of dark-toned, reddish brown, calibrated Large tsavorite rough is extremely rare, so there’s a huge pyrope-almandine garnets are used in inexpensive, mass- increase in value between small and large fashioned stones. market jewelry. Tsavorite is colored by vanadium, and the best is pure green or Color-change garnets are usually pyrope-spessartine mixtures green with a slight amount of blue. with vanadium as a coloring agent. Tsavorite is mined mostly in East Africa. Many historically important red gems, long identified as ruby, were recognized as spinel in the 1800s. Demantoid, the transparent green variety of andradite, is colored by a mixture of chromium and iron. In spinel, the higher the chromium content, the stronger the red hue. Demantoid comes mostly from sources in Russia and Africa. Inconsistent supply and continuing demand for demantoid Red is the most rare and valuable spinel hue. keep prices high. Spinel forms in the same metamorphic rocks as corundum, New discoveries of spessartine in the 1990s made it much and both gem species are often found in the same alluvial more available. deposits. Spessartine is the most valuable orange garnet, and large, Because spinel is rare and synthetic spinel is common, many fine-quality stones sell at a premium. people have never seen the natural stone. Key Terms Hollowback—A cabochon with a curved, undercut Isomorphous replacement—Substitution of one chemical bottom, designed to lighten the tone of the gem material. element for another in the crystal structure of a mineral. 161