The Lustre Primer
Mineralogists read shine like a label: the named lustres are a quick diagnosis made long before any instrument is reached for.
The first and sharpest division is metallic against non-metallic. Metallic minerals — pyrite, galena, native copper — reflect strongly because free electrons at the surface re-emit incident light; they stay opaque in thin fragments and give a dark, dense streak. Everything else falls into the non-metallic family.
Within that family, brilliance is graded. At the top stands adamantine lustre, named for diamond, whose refractive index near 2.4 reflects an exceptional share of light at every face. Below it sits vitreous lustre, the ordinary glassy shine of quartz and calcite — the unmarked baseline against which the others are noticed.
On narrow screens, swipe or scroll the plate sideways.
Internal structure writes its own signatures. Pearly lustre, seen in talc and muscovite, comes from light reflecting off stacked cleavage planes, so the glow seems to lie just beneath the surface. Silky lustre belongs to fibrous aggregates: satin spar gypsum draws a band of sheen along its fibres exactly as silk cloth does along its threads.
At the quiet end are resinous lustre, the warm low shine of amber and much sphalerite; greasy lustre, the filmed look of nepheline; and dull or earthy lustre, the near-absence of reflection in kaolinite and bauxite, where surface scatter swallows almost everything.
One caution belongs with the vocabulary: lustre is judged on a fresh surface. Tarnish can rewrite it — bornite weathers into iridescent 'peacock ore' — and a weathered rind can make a vitreous mineral look dull. The named lustres describe what a clean break shows.
Further reading