Types of Gemstone Lustre: Identification Guide
Lustre is the quality and intensity of light reflected from a mineral's surface. It is determined primarily by the mineral's refractive index and its ability to take a polish. Mineralogists recognize seven main lustre types for gemstones: adamantine, vitreous, pearly, silky, resinous, waxy, and metallic. Lustre is one of the first properties to assess when identifying an unknown stone.
How Lustre Is Assessed
Lustre is observed under normal reflected light, not transmitted light. You assess the surface of the stone — not its transparency — by watching how it reflects a light source. The refractive index (RI) is the underlying physical cause: the higher the RI, the more reflective the surface, and the brighter and more mirror-like the lustre. Adamantine lustre (diamond) corresponds to a very high RI; vitreous lustre (quartz) corresponds to a middle-range RI.
Lustre can vary on different faces of the same crystal, and it may differ between a freshly broken surface and a polished one. These are normal mineralogical characteristics, not defects.
The Seven Main Lustre Types
Adamantine Lustre
Adamantine is the highest non-metallic lustre. Minerals with adamantine lustre are intensely reflective, approaching a mirror finish, and appear to glow from within when lit. It corresponds to very high refractive indices.
Diamond is the defining example. Corundum (ruby, sapphire) can approach subadamantine. Cassiterite, cerussite, and zircon also exhibit adamantine to subadamantine lustre.
Vitreous (Glassy) Lustre
Vitreous lustre — from the Latin vitrum (glass) — describes a surface that reflects light like a clean pane of glass. Approximately 70% of all known minerals have the potential to exhibit vitreous lustre, making it by far the most common type.
Vitreous lustre is associated with moderate-to-high refractive indices in the range typical of silicate and oxide minerals.
Examples on this site: amethyst, clear quartz, emerald, sapphire, ruby, garnet, black tourmaline.
Pearly (Nacreous) Lustre
Pearly lustre is a soft, iridescent surface reflection similar to the surface of a pearl or the inner layer of a mollusk shell. It appears on minerals with perfect or near-perfect cleavage parallel to a reflective surface — thin layers within the mineral reflect and interfere with light, producing the soft iridescent glow.
Examples: moonstone (whose adularescence is an extreme form of pearly effect within the stone), selenite (gypsum cleavage surfaces).
Silky Lustre
Silky lustre is produced by minerals composed of parallel fibers or elongated crystals tightly bound together. The parallel structure reflects light similarly to a bundle of silk thread, producing a soft, directional sheen.
Examples: tiger's eye (fibrous quartz replacement of crocidolite), satin spar gypsum (a fibrous form of the same mineral as selenite). Tiger's eye's chatoyancy — the cat's eye band of light — is a direct expression of its silky internal structure.
Resinous Lustre
Resinous lustre resembles the surface of dried tree resin or amber — a warm, slightly yellowish, non-metallic shine without the sharpness of glass. It tends to appear in softer minerals and mineraloids with lower refractive indices.
Amber (fossil resin) is the defining example. Some garnets — especially almandine and hessonite — exhibit resinous to vitreous lustre depending on the specimen. Sphalerite can be resinous to adamantine.
Waxy Lustre
Waxy lustre describes a surface appearance resembling a polished candle, beeswax, or paraffin — a soft glow without sharp highlights. It appears in minerals with low refractive indices that do not take a high polish, or in cryptocrystalline and massive minerals where the surface is microscopically rough.
Examples: turquoise, jade (both nephrite and jadeite), opal (rough or poorly polished surfaces).
Metallic Lustre
Metallic lustre is the high, opaque reflectivity of metals and metallic minerals. Light does not penetrate the mineral at all; the surface acts as a mirror. Metallic minerals are opaque in all but the thinnest sections.
Examples on this site: hematite (when polished to a specular finish), pyrite. Rainbow hematite shows iridescent metallic lustre from thin-film interference.
Lustre Type Reference Table
| Lustre | Appearance | Refractive index | Examples on site |
|---|---|---|---|
| Adamantine | Mirror-like, intense brilliance | Very high (>1.9) | Diamond |
| Vitreous | Glass-like shine | Moderate to high (1.5–1.9) | Amethyst, emerald, sapphire, garnet |
| Pearly | Soft iridescent nacreous glow | Moderate, layered cleavage | Moonstone, selenite |
| Silky | Directional thread-like sheen | Varies; structure-dependent | Tiger's eye |
| Resinous | Warm amber-like glow | Lower (1.4–1.6) | Amber, almandine garnet |
| Waxy | Soft candle-wax surface | Low to moderate | Turquoise, jade, opal |
| Metallic | Opaque mirror | Extremely high or absorbent | Hematite, pyrite |
Lustre in Gem Identification
Lustre is a useful first filter when identifying an unknown stone. A strong metallic lustre immediately suggests a different class of mineral than a vitreous or waxy surface. Combined with hardness and color, lustre narrows the field significantly.
Lustre can also reveal treatments: some surface coatings applied to enhance a stone's appearance produce a lustre inconsistent with the base mineral. A stone sold as "natural" turquoise with a perfectly vitreous shine, for example, may have received a resin coating — turquoise's natural lustre is waxy, not glassy.
For context on how lustre relates to mineral classification and gemstone identification more broadly, see gemstone vs crystal vs mineral and browse the gemstones directory, which filters stones by lustre type.