Liddicoat, Richard T., Jr. Gemological Institute of America, Los Angeles, California.
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A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often interlayered with white, but is also commonly composed of various shades of gray and white. Since agate is relatively porous, it can be dyed permanently in red, green, blue, and a variety of other colors. The difference in porosity of the adjacent layers permits the dye to penetrate unevenly and preserves marked differences in appearance between layers. The properties of agate are those of chalcedony: refractive indices 1.535 and 1.539, hardness 6.5 to 7, and specific gravity about 2.60.
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