Greasy Lustre
Greasy Lustre
The soft, oily sheen that distinguishes nephrite, chrysoprase, and related minerals
Greasy lustre — also termed oily lustre — is a surface optical quality in which reflected light is diffused across a mineral face in a manner that evokes the appearance of a thin film of oil or grease. Rather than producing the sharp, mirror-like reflection of a vitreous or adamantine surface, greasy lustre yields a soft, somewhat muted sheen that appears to sit just beneath the surface of the stone. It is one of the classical lustre categories codified in standard mineralogical literature, described in authoritative references including Hurlbut and Klein's Manual of Mineralogy, and remains an important diagnostic property in the identification of nephrite jade, chrysoprase, and several serpentine-group minerals.
Physical Basis
Lustre in minerals is governed principally by two variables: the refractive index (RI) of the material and the micro-scale texture of its surface. Vitreous lustre, the most common type among transparent gemstones, arises when a smooth, homogeneous surface with a moderate RI (roughly 1.45–1.90) reflects light in a relatively coherent, specular manner. Greasy lustre emerges when this coherence is disrupted — either by a surface that is microscopically uneven, by a slightly porous or fibrous internal structure that scatters light back through the surface layer, or by a combination of both factors.
In practical terms, the surface of a greasy-lustre mineral does not present a single flat reflective plane at the microscopic level. Minute undulations, interlocking crystallite boundaries, or fine porosity scatter incident light across a range of angles rather than returning it as a single specular highlight. The result is a broad, diffuse glow rather than a point reflection — the visual analogue of light on a lightly oiled wooden surface as opposed to light on polished glass.
Refractive index also plays a role. Materials with RIs in the range of approximately 1.50–1.65 that lack the internal ordering needed for strong specular reflection tend to fall into the greasy category rather than the vitreous one. Where the RI is higher and the surface is smooth, lustre grades toward resinous or adamantine; where the RI is lower and the surface is rough, it grades toward waxy or dull. Greasy lustre occupies a specific intermediate zone defined as much by surface micro-texture as by RI alone.
Principal Gemstone Examples
Nephrite jade is the most widely cited example of greasy lustre in gemmological literature. Nephrite is an aggregate of interlocking tremolite–actinolite amphibole fibres, and it is precisely this fibrous, felt-like microstructure that produces the characteristic oily surface sheen. When nephrite is polished, the interlocking fibres do not yield a single coherent reflective plane; instead, light is scattered by the countless fibre boundaries just beneath the surface, producing the soft, almost liquid glow that has been prized in Chinese and Maori lapidary traditions for millennia. The greasy lustre of nephrite is considered one of its defining aesthetic virtues and is a key point of distinction from jadeite, which typically exhibits a higher, more vitreous lustre on a well-polished surface.
Chrysoprase, the apple-green to mint-green chalcedony coloured by nickel, similarly displays greasy to waxy lustre. Chalcedony in general is a cryptocrystalline aggregate of quartz, and its submicroscopic grain structure — individual crystallites measured in nanometres — prevents the development of a clean specular surface even after careful polishing. The lustre of chrysoprase is sometimes described as waxy rather than greasy, and the two terms do overlap in practice; the distinction is largely one of degree, with greasy implying slightly more diffuse reflection and a marginally higher perceived depth than waxy.
Serpentine-group minerals, including antigorite and lizardite, also commonly exhibit greasy lustre on massive or fibrous surfaces. The layered, sheet-silicate structure of serpentine minerals produces a surface that scatters light in a manner broadly similar to nephrite, though serpentine lacks the toughness and the cultural prestige of true jade.
Other minerals that may display greasy lustre include massive fluorite on cleavage-free surfaces, certain massive garnets, and some varieties of obsidian on curved fracture surfaces — though in the last case the term resinous is sometimes preferred.
Greasy Lustre as a Diagnostic Property
In practical gemmological identification, lustre is assessed on a polished surface under a direct light source — ideally a fibre-optic or focused LED — held at a low angle to the stone. The observer notes the character of the primary specular highlight: its sharpness, its apparent depth, and whether it appears to float on the surface or to emerge from within the material. A greasy lustre presents a highlight that is broad, soft-edged, and seemingly embedded just beneath the outermost surface layer, in contrast to the crisp, surface-level highlight of a vitreous stone such as quartz or topaz.
Greasy lustre is particularly useful in distinguishing nephrite from its simulants. Serpentine (bowenite), soapstone, and aventurine quartzite are among the materials sometimes substituted for nephrite in the trade; their lustre characters — while sometimes superficially similar — differ in detail. Bowenite serpentine tends toward a slightly more vitreous quality on a fine polish, while soapstone is decidedly pearly to greasy in a manner that lacks the depth of nephrite. Aventurine quartzite, with its quartz grain boundaries, tends toward vitreous to waxy rather than truly greasy.
It should be noted that lustre assessment is inherently somewhat subjective and is best used in conjunction with other properties — refractive index, specific gravity, spectroscopic data, and microscopic examination — rather than as a standalone diagnostic criterion. Nevertheless, a pronounced greasy lustre on a green, dense, tough material is a strong indicator pointing toward nephrite, and its presence or absence is routinely noted in laboratory reports from institutions such as GIA and Gübelin.
Lustre Terminology in Context
The classical lustre scale used in mineralogy and gemmology recognises a spectrum of types, from the highest reflectivity to the lowest: adamantine (diamond-like), subadamantine, vitreous (glassy), resinous, greasy, waxy, pearly, silky, and dull (or earthy). These categories are not rigidly bounded; a single specimen may exhibit lustre that sits between two descriptors, and different faces or zones of the same stone may display different lustre characters depending on surface preparation and microstructure.
Greasy lustre sits between resinous and waxy on this scale. Resinous lustre — characteristic of amber, sphalerite, and some garnets — implies a slightly higher reflectivity and a warm, amber-like quality. Waxy lustre — seen in turquoise, chalcedony, and some serpentine — implies a slightly lower reflectivity and a surface that appears more matte. Greasy occupies the zone where reflectivity is moderate but diffusion is high: enough light is returned to create a perceptible sheen, but the scattering is sufficient to prevent any mirror-like quality.