The standard color is a clear deep-red, and such a spinel is known in the gem trade as a _spinel-ruby_. If the color is rose-red, it is a _Balas ruby_; if orange, it is _rubicelle_, if of a violet tinge, _almandine_. When small quantities of other elements replace the magnesium, the color is greatly changed. For example a little iron present gives the crystals a dark-green to black color, and the spinel is known as _ceylonite_. If there is both iron and chromium present, the color becomes yellowish or greenish-brown, and this variety is _picotite_. When the impurities are iron and copper, the color becomes grass-green, and it is called _chlorospinel_. A form, in which the magnesium is completely replaced by iron, is black in color and termed _hercynite_, and occurs fairly abundantly in Westchester Co., N. Y. From Amity, N. Y., to Andover, N. J., there is a belt of granular limestone in which spinel of all colors is found. St. Lawrence Co., N. Y., is also a rich locality. Bolton, Mass., Newton, Sterling, and Sparta, N. J., North Carolina, Alabama, and California all yield spinel. Magnesite MgCO₃ Occurs in cleavable or compact porcelain-like masses; hardness 4; specific gravity 3.1; color white to gray; luster vitreous; translucent on thin edges. Magnesite is white and brittle, and cleaves perfectly parallel to the faces of the rhombohedron, but it seldom occurs in crystals. It will effervesce in warm hydrochloric acid and has some resemblance to calcite, but can be distinguished by the greater hardness. It is still more like dolomite, both having the same color and cleavage, both effervescing in warm hydrochloric acid; but the magnesite has half a point greater hardness and the porcelainous appearance. Magnesite is used in toilet preparations, paper making, and mixed with asbestos, as a covering for heating pipes. Magnesite is found in Massachusetts, Pennsylvania, Texas, and in large deposits in California and Washington. Dolomite (MgCa)CO₃ Pl. 19 & 29 Occurs in crystals, or in cleavable or granular masses; hardness 3.5; specific gravity 2.8; color white to pink or gray; streak white; luster vitreous; transparent on thin edges. Dolomite crystallizes in the hexagonal system, in rhombohedrons (hemihedral form), which are more or less modified by faces on the corners or edges. The cleavage is parallel to the rhombohedron, and it will effervesce in warm hydrochloric acid. Sometimes the crystal faces are curved, and when this is the case, dolomite is easily determined. Usually however dolomite resembles both calcite and magnesite. From the calcite it is distinguished by the greater hardness, and from magnesite by lesser hardness and not being porcelainous in appearance. Some of the commoner forms are shown on Plate 29, crystals like C being found embedded in anhydrite and gypsum.