Luster is the ability of minerals to reflect light from their surface. It depends upon various factors, particularly on the number of rays absorbed or reflected, and on the character of the mineral surface (ie even or rough). The luster of a mineral increases for these minerals with a high refractive index, and becomes less intense with a higher light absorption, and a rough surface, but it is not dependent on the color of the mineral.

Luster is observed in daylight on even, unweathered and clean surfaces. A particular mineral does not always have the same luster. In granular aggregates the luster is less evident than in individual crystals. For instance, magnetite has a metallic luster on its crystal faces but a dull luster in a granular mass. The luster of granular minerals can be studied under magnification or by means of a microscope.

Cleavage planes have a brighter luster than crystal faces. It is sometimes very difficult to determine the luster of dark minerals. However, an experienced collector can specify the type of luster fairly reliably by comparing other selected types of luster.

The types of luster to be distinguished are as follows:

Metallic:

Characteristic of the majority of opaque minerals. It is best seen on freshly fractured surfaces and cleavage planes, e.g. Galena, chalcopyrite or magnetite.

Sub-metallic:

Occurs on transparent or semi-transparent (sub transparent) minerals with a refractive index of 2.6 - 3.0, such as cinnabar and cuprite.

Adamantine:

Occurs only on transparent and translucent minerals with a refractive index of 1.92 and over, because of the total reflection of light, such as cerussite, zircon and diamond. (Shining).

Vitreous:

Resembles the luster of glass. It is typical of transparent as well as translucent minerals with a refractive index of 1.3 - 1.9, such as fluorite, quartz and corundum.

Greasy:

Self-explanatory. This is shown particularly by minerals displaying numerous microscopic inclusions, such as opal and cordierite.

Pearly:

Typical of transparent or semi-transparent minerals with a perfect cleavage, such as gypsum, muscovite or stilbite.

Silky:

Particuliar to minerals having a parallel, fine-fibrous structure, such as asbestos and crocidolite.

Dull:

Typical of minerals of an earthy character, such as kaolinite and pyrolusite. (Glimmering).