Mineral Luster: Vitreous, Metallic, Pearly, Adamantine and More

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What is meant by luster

The luster of a mineral is its appearance when light reflects off its surface. It is described by comparison with a familiar object: glass, a metal, mother-of-pearl, silk, wax. It is one of the basic criteria for identifying a mineral, along with hardness, density, streak and cleavage.

Luster is easy to confuse with three other notions. It is not colour (a pyrite is brass-yellow, but its luster is metallic). It is not transparency (see transparency and translucency). Nor is it polish: a smooth surface shines more, but luster is a property of the material, assessed on a fresh fracture or on a natural face.

The main lusters and their examples

Luster	Appearance	Examples
Metallic	Bright like polished metal, opaque mineral	Pyrite, galena, magnetite, specular hematite
Submetallic	Close to metal, less bright	Cuprite, cinnabar, dull hematite
Adamantine	Intense, “hard” brilliance of diamond	Diamond, zircon, cerussite, cubic zirconia
Vitreous	Luster of glass	Quartz, topaz, beryl, tourmaline, calcite, fluorite
Resinous	Luster of resin or gum	Amber, sulfur, some sphalerites
Greasy	Surface that seems oiled	Common opal, massive quartz, sometimes nephrite
Waxy	Luster of wax, soft	Turquoise, chalcedony, jade, serpentine
Pearly	Reflections of mother-of-pearl, in thin layers	Muscovite, talc, stilbite, cleavage faces of gypsum
Silky	Reflections of parallel fibres	Fibrous gypsum (satin spar), fibrous malachite, tiger's eye
Dull (matte, earthy)	Almost no shine	Kaolin, limonite, chalk

The types blend into one another, and authors sometimes describe the same mineral slightly differently: a sphalerite may be adamantine, resinous or submetallic depending on the specimen.

Why one mineral shines more than another

The brightness of a surface depends on the share of light it reflects. For light arriving head-on at a smooth surface, the proportion reflected is given by the Fresnel formula: R = ((n − 1) / (n + 1))², where n is the refractive index of the material. The larger n is, the more the material reflects. Here are the calculated results:

Material	Index n (approx.)	Light reflected (face)	Luster
Fluorite	1.43	3.2%	Vitreous
Ordinary glass	1.52	4.3%	Vitreous
Quartz	1.55	4.7%	Vitreous
Corundum (ruby, sapphire)	1.77	7.7%	Bright vitreous
Zircon	1.92 to 1.98	about 10%	Adamantine
Diamond	2.42	17.2%	Adamantine
Moissanite (synthetic)	2.65 to 2.69	about 21%	High adamantine

According to common textbooks, luster is vitreous up to an index of about 1.9, adamantine between about 1.9 and 2.6, submetallic between about 2.6 and 3.0, and metallic beyond that for opaque minerals. These limits are guidelines, which authors place slightly differently. For metals, what matters is light absorption and the presence of free electrons.

Luster and surface: what can mislead

• Natural faces and fracture. Quartz has a vitreous luster on its faces and a greasier look on its fracture. Always look at a fresh surface, neither weathered nor dusty.
• Polish and wax. Careful polishing, or a layer of oil, wax or varnish, gives a surface shine that does not belong to the stone itself. Many tumbled stones are oiled for sale.
• Metallic coatings. “Aura” or “titanium” quartz is coated with a thin layer of metal deposited under vacuum: its iridescent sheen is artificial. See aura quartz.
• Patina. A pyrite exposed to humidity for a long time tarnishes and may crumble; a rough stone can have a very different luster on the inside.

Optical effects not to be confused with luster

Some phenomena look like a particular luster, yet they come from the internal structure:

Effect	Cause	Examples
Chatoyancy	Parallel fibres or needles that reflect light in a band (visible on a cabochon)	Tiger's eye, cat's eye
Asterism	Rutile needles aligned in three directions, forming a star	Star sapphire or star ruby
Adularescence	Light scattered by fine internal lamellae	Moonstone
Labradorescence	Interference on feldspar lamellae	Labradorite, spectrolite
Aventurescence	Flakes or inclusions that glitter	Aventurine, sunstone
Play of colour	Diffraction by ordered silica spheres	Precious opal
Iridescence	Interference in a thin film or a fracture	Iridescent quartz, hematite, bornite

See also tiger's eye, labradorite and opal.

How to observe luster without equipment

1. Choose a single light source (a lamp, or a window), away from any stray reflections.
2. Clean the surface with a dry cloth; avoid oil from your fingers.
3. Hold the stone at arm's length and turn it slowly: true luster persists on a face, while an optical effect such as chatoyancy or adularescence moves.
4. Compare it with a familiar object: a glass, a metal spoon, a mother-of-pearl button, a piece of silk.
5. Write down the word you chose and the condition of the surface (natural face, fracture, polish).

When luster helps identify a stone

Luster is a good first clue, never proof. A classic example is “fool's gold”. Pyrite and native gold both have a metallic luster and a yellow colour, but gold is about four times denser (19.3 against about 5), bends without breaking, and stays the same yellow from every angle, whereas pyrite is brittle and has a more brassy sheen. Likewise, a “diamond” whose luster looks less intense than a real one proves nothing: badly cut, a diamond can look dull; well cut, glass can surprise. For a valuable piece, see labels and certificates. For simple tests, see real or fake stone.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.