luster: one construct, many systems

In mineralogy, luster is the characteristic appearance of a mineral's surface in reflected light, described by named categories such as metallic, vitreous, resinous, pearly, silky, greasy, and adamantine. It is a diagnostic property fixed by the mineral's refractive index, absorption, and surface structure rather than by its colour. Klein and Hurlbut's Manual of mineralogy treats luster as a standard property used to identify and classify minerals. The referent is the quality of light a solid surface returns, and it is the same underlying optical construct that collectors read on a coin or a pearl.[1]

Unit

The qualitative category of reflected-light appearance of a surface

Recovery means

Not applicable

Measured by

Classified by eye into named luster categories, correlated with refractive index and surface condition

In gemology, lustre is the surface brilliance of a gem or pearl, and for pearls in particular it is the sharpness and depth of the reflections produced by light bouncing off the overlapping nacre layers, the single most important factor in a pearl's quality. Read's Gemmology treats lustre, and the related pearl property of orient, as key appearance characteristics. It is the same construct as mineral or coin luster (the quality of reflected-light appearance of a surface), applied here to graded gem material.[2]

Unit

The sharpness and depth of reflected-light brilliance from a gem or pearl surface

Recovery means

Not applicable

Measured by

Assessed by the clarity and intensity of surface reflections, especially the quality of nacre reflections in pearls