PRECIOUS STONES	19
	There is also much confusion as to the difference between sard and sardonyx, although the name is suggestive. The sard is a solid color stone similar to the carnelian, only of a darker, brownish-red color; the sardonyx is the same stone with underlying strata of a lighter or other colors, similar to the onyx. As many of the features by which a mineral is recognized in the natural state are obliterated in the cutting, and the tests for hardness and specific gravity are not always possi­ble to the dealer, he must rely very largely on the instinctive judgment which comes of familiarity, and a few simple facts which serve to guide him. A chapter has been devoted to the differences existing between stones similar in color and general appearance when cut, and tables appended, showing the various methods by which their identity may be estab­lished.