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Lustre (British English) or luster (American English; see spelling differences) is the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin lux , meaning "light", and generally implies radiance, gloss, or brilliance.
For example, anatase is less hard (5.5–6 vs. 6–6.5 on the Mohs scale) and less dense (specific gravity about 3.9 vs. 4.2) than rutile. Anatase is also optically negative, whereas rutile is optically positive. Anatase has a more strongly adamantine or metallic-adamantine luster than that of rutile as well. [8]
The luster of a diamond is described as "adamantine", which simply means diamond-like. Reflections on a properly cut diamond's facets are undistorted, due to their flatness. The refractive index of diamond (as measured via sodium light, 589.3 nm) is 2.417. Because it is cubic in structure, diamond is also isotropic
The former is the light reflected from the surface of the stone—its luster. Diamond's adamantine ("diamond-like") luster is second only to metallic (i.e., that of metals); while it is directly related to RI, the quality of a finished gem's polish determines how well a diamond's luster is borne out.
The metallic luster of the minerals of the group resembles the luster of the surface of a fresh fracture of metals. It is clearly visible only on the non-oxidized surface of the sample. Minerals that have a metallic luster are, as a result, opaque and heavier than minerals that have a non-metallic luster.
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Transparent zircon is a well-known form of semi-precious gemstone, favored for its high specific gravity (between 4.2 and 4.86) and adamantine luster. Because of its high refractive index (1.92) it has sometimes been used as a substitute for diamond, though it does not display quite the same play of color as a diamond.
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