Lustre

What is lustre?

Lustre refers to the perceived texture and reflectivity of a surface under reflected light. In the case of minerals it is influenced both by the internal properties of the crystal - its chemistry and atomic structure - and by the roughness and structure of its surface. Lustre thus has links both to the colour of minerals and to what's known as their crystal habit (the shapes and structures that they commonly form). It is also dependent on the transparency (diaphaneity) of the minerals and their refractive index - the angle at which light rays bend as they enter the crystal.

For more information on lustre, a useful source is this page on Mindat

Describing mineral lustre

As with colour, lustre is not a strongly diagnostic property, because many minerals have the same lustre, and many show a range of lustres depending on the purity and physical structure of the specimens. Nevertheless, describing lustre can help to narrow down the possible identities of the mineral, especially when combined with other properties such as colour, transparency (diaphaneity) and mineral habit.

Like colour, lustre is not measured in any quantitative way, but qualitatively by assigning the mineral to a category. The categories used to define lustre, however, are somewhat arbitrary and the divisions between them blurred, so classification is often approximate. Different faces of the mineral may also reveal different lustres. Moreover, a number of different and rather ambiguous terms have been developed to describe lustre, many of them difficult to interpret or apply. For the amateur, therefore, simplicity is a good principle, and it is often advisable to think of lustre in broad classes, rather than as a narrow and precise definition.

To minimise these uncertainties in classifying lustre, it's important to standardise the conditions in which the specimen is examined. Generally, this means ensuring that the specimen is clean, dry and viewed in good lighting conditions. It is also useful to move or rotate the specimen so that it can be seen under both direct and indirect light, for this helps to observe the reflections and textures.

The figure to the right identifies the eleven main terms that are commonly used for minerals and shows how they relate to two of the primary dimension of lustre - brightness and texture (the third is transparency).

What this diagram emphasises is how much these terms overlap and how crowded the space is within the centre of the scale. Add in the fact that lustre may vary from face to face on the mineral and between different specimens of the same mineral and you'll understand that it's not worth agonising too much about your precise classification of lustre.

More detail on the eleven classes and an example of a mineral in each class is given belo.

The eleven classes of lustre

Adamantine
Adamantine lustre refers to the property of bright reflectivity and sparkle that is seen in a few, generally transparent minerals. is best seen by rotating the mineral slowly so that the light plays at different angles to create brief flashes of reflected light (see video to right). As in this video, it is most readily apparent in gem-quality minerals, and cutting of these gems is partly done to enhance this lustre. In many natural minerals adamantine lustres are somewhat more muted, and the term sub-adamantine is sometimes used.

Adamantine lustre is a notable characteristic of diamond, but is also seen in other minerals, including some specimens of zircon, cerrusite, sphalerite, cassiterite and rutile.

Vitreous (Glassy)
Vitreous (or glassy) lustre is the most common form of lustre amongst non-metallic minerals, and is found in about seventy per cent of minerals overall. It's characterised, as the name implies, by a glassy reflection and refraction, and thus occurs mainly in transparent or translucent crystals. However, it also occurs in some opaque minerals, including a few metallic ones.

Compared to adamantine lustre, vitreous lustre is rather 'flatter' and less sparkly or bright. The flash of reflection when the specimen is rotated is usually seen across a whole surface rather than as a bright spot. As the lustre becomes less bright and glassy, and grades into resinous or pearly lustres, it is often referred to as sub-vitreous.

Metallic
Metallic lustre is recognised as a surface sheen like polished metal. As such, it is somewhat like a vitreous lustre, but without the transparency. It can, however, vary greatly in the degree of reflectivity observed, in which case it may be referred to as sub-metallic.

As might be expected, metallic lustres are mainly found in metals, and are common for example in gold, galena, tungstenite and chalcopyrite. The example of pyrite in the image to the right shows a strong metallic lustre.

Resinous
Resinous lustre appears as a somewhat toned-down and dulled version of vitreous lustre,
and shows a muted reflection, but a relatively clear transparency. It is sometimes likened to the character of smooth and translucent plastic. It's often seen in red brown, yellow or red minerals that are translucent, such as some garnets and sphalerite. However, it's most clearly seen in amber - though this is not a true mineral but a form of petrified resin (see image, right)

Pearly
Pearly lustre, as its name implies, has the look of mother of pearl. It tends to be associated with minerals that are translucent to almost opaque, often with a somewhat cloudy appearance, but with a slight surface sheen. In more translucent minerals such as muscovite (see image, right), the pearliness may originate from the presence of closely stacked cleavage lines which cause the reflections to seem rather blurred. In these cases, however, the pearly lustre shows only on the smooth crystal face parallel to a cleavage plane. On other faces, the lustre may be vitreous. Pearly lustre is commonly seen on mica, calcite and feldspar. .

Silky
Silky lustres appear as an almost smooth surface with a slight sheen, like fine silk cloth, and like silk they often they have a minutely fibrous appearance, as if made up of thin threads. This is due to the structure of the mineral, which is composed of long, fine and parallel crystals, like those seen in tremolite (image, right). Other minerals that show silky lustre include some forms of gypsum, malachite and chrysotile (a form of asbestos).

Greasy and oily
Greasy and oily lustres are similar to each other and have the appearance that their names imply. They are typically found in weakly translucent to opaque minerals to which they give a dull surface sheen somewhat like an unsugared fruit pastille. They are seen in a number of mineral species, including serpentine, wulfenite and some zircons. The example shown to the right is a specimen of lapis lazuli (which in reality is not a mineral but a type of rock since it is composed of a mix of minerals).

Waxy
Waxy lustre is similar to oily or greasy, but with a duller sheen. It has the appearance of candle wax or, for those of advanced years, a piece of lard, and is usually found on minerals that are almost or completely opaque. Amongst other minerals, it occurs on talc, chrysocolla and some forms of opal. Serpentine also has a waxy lustre, is shown in this image (right) of serpentine in the Cobb Valley.

Dull and Earthy
Dull and earthy lustres are those which lack significant reflectivity or sheen and thus have a matte or rough surface. The lustre is referred to as dull when the surface is relatively smooth, and earthy when it is rough such that individual grains or clusters of grains are evident. These lustres are typical of clay minerals such as kaolinite and montmorillonite)and also many metals when found in granular form. Some non-metals, too, show a dull or earthy lustre in some forms - for example azurite and carnotite (image, right). An earthy lustre is also characteristic of limonite, which is not a true metal but a pseudomorph or iron oxides and hydroxides, and which commonly forms as a result of the oxidation of iron (image, right).

Adamantine lustre in a cut specimen of white sapphire