A mineral's luster is the general appearance of its surface in reflected light. There are two broad types of luster: metallic and nonmetallic. Metallic luster is that of an untarnished metal surface, such as gold, steel, copper, galena, pyrite, and hematite. Minerals with metallic luster can also be described as having a "shiny", "dull", or "iridescent" luster. For example, the pyrite mineral shown in the left photo has mostly a shiny, metallic luster. Minerals of metallic luster are opaque to light, even on thin edges. By contrast, minerals with non-metallic luster are generally lighter in color and show some degree of transparency or translucency, even though this may be on a thin edge.
There are a number of terms that describe non-metallic luster, such as vitreous (having the luster of a broken piece of glass); adamantine (having the brilliant luster of a diamond); resinous (having the luster of a piece of resin); pearly (having the luster of a pearl or mother-of-pearl); greasy (appearing to be covered with a thin layer of oil); silky (appearing as the surface of silk or satin); dull (producing little or no reflection); and earthy (having a non-lustrous appearance of raw Earth. The quartz minerals shown in the left photo represent both opaque crystal quartz and a variation of quartz called amethyst. They both have a vitreous appearance.
The photo to the left represents rose quartz. Note that this particular one has a "greasy appearance". Rose quartz may also have a vitreous appearance, therefore it takes a little practice to notice the subtle differences between vitreous luster and greasy luster. Calcite may also have a vitreous or greasy luster. In fact, all silicate minerals will generally have one or the other of the two. For example, when comparing the two minerals in their photos, both appear to resemble glass; however, the rose quartz has the shiny-but-blurry appearance as if had been covered with a thin layer of oil.
Now this microcline feldspar to the left has a non-reflective appearance, thus displaying a dull luster. More than often, distinguishing dull luster from earthy luster can be difficult for beginners. Earthy luster, represented in the next photo, has a granular appearance; whereas, dull luster does not. Recall that metallic luster can also have a dull appearance and therefore be classified as having a dull metallic luster. However, it is clearly evident that this microcline feldspar does not have a metallic appearance. In some cases, a mineral may have a combination of two non-metallic lusters. If this should be the case, the one most dominant will be classified.
This azurite mineral has a granular texture with a freshly broken appearance; therefore, it displays an earthy luster. In some cases, earthy minerals look like dirt or dried mud, while others may be rough and porous in texture. Earthy luster may also look like unglazed pottery. Other minerals which may display earthy luster include malachite, kaolinite, pyrolusite, limonite, and bauxite. In such minerals, relatively little light is reflected from the surface and they lack the shiny appearance of a metallic or glassy appearance.
Resinous minerals have the appearance of resin, chewing gum or smooth-surfaced plastic. A common mineral that represents a resinous luster, such as the one in the left photo, is amber. Amber is simply a form of fossilized resin that sometimes contains insects, very small lizards, and pollen/spores. Most often, these type of minerals have a yellow, dark-orange or brown color which resembles honey, but not necessarily the same color. Minerals with resinous luster can show a transparent, translucent, or opaque reflectivity. Other minerals that display resinous luster may include sulfur, anglesite, and wulfenite.
Minerals, such as barite (left photo) exhibit a luster similar to the inside of a mollusk shell. Many micas display a pearly luster, and some minerals which have a pearly luster also have an iridescent hue. In some cases, minerals, such as calcite and aragonite may display a pearly luster on cleavage cracks parallel and below the their reflecting surface. Many pearly minerals consist of thin, transparent co-planar sheets. Light reflecting from these layers give them a luster resembling that of a pearl. Pearly luster is a dominant characteristic for opals and moonstones.
Silky luster is produced by a mineral's shiny, fibrous body appearance. Minerals with silky luster possess microscopic inclusions (very fine fibrous structures), causing them to display similar optical properties of silk cloth. Such fibrous structures may occur within the mineral and/or aggregates on its surface. This type of luster is best seen on rough specimens of tiger's eye. However, when polished, tiger's eye exhibits a silky-vitreous luster known as "silky sheen" luster. Other examples include asbestos, aragonite, satin-spar gypsum, and actinolite (left photo).
Adamantine minerals possess a brilliant, superlative luster which is most notably observed in diamonds. Such minerals are transparent or translucent. Minerals with "true" adamantine luster are uncommon and thus are more valuable. Those minerals exhibiting less luster may still be of value; however, they are referred to as subadamantine minerals. The two crystals in the left photo are known as Herkimer Diamond Quartz. Due to their brilliant appearance and high degree of quality, they are commonly known for displaying adamantine luster.
As the name implies, waxy minerals have a luster resembling the surface of a wax candle or beeswax. Examples include jade, carnelian, chalcedony (left photo), opal and turquoise. Such minerals have a translucent to opaque reflectivity and may come in a variety of colors. Waxy luster resembles greasy and resinous luster; however, the surfaces on which it is seen are more irregular. The chalcedony mineral (left photo) has formed in globular aggregates, resembling a bunch of grapes. This type of crystal habit is referred to as botryoidal. Many waxy luster minerals possess such form.