Luster describes how a mineral appears to reflects light, and how
brilliant or dull the mineral is. The terms used to describe luster are:
- Metallic (also known as splendent)
- Submetallic
- Vitreous (also known as glassy)
- Adamantine (also known as brilliant or
diamondlike)
- Resinous (also known as resinlike)
- Silky
- Pearly (also known as mother-of-pearl)
- Greasy (also known as oily)
- Pitchy (also known as pitchlike)
- Waxy (also known as waxlike)
- Dull (also known as earthy)
Metallic - Minerals with a metallic luster are opaque and reflective, like metal.
The metallic elements, most sulfides, and some oxides belong in this category.
Submetallic -
Describes a mineral that is opaque to nearly opaque and reflects well. Thin splinters or sections of submetallic
minerals are translucent.
Vitreous - This
luster accounts for roughly 70 percent of all minerals. Minerals with a vitreous luster
have reflective properties similar to glass. Most of the silicates, carbonates, phosphates, sulfates, halides, and hydroxides have a vitreous luster.
Adamantine - Transparent
to translucent minerals with a high refractive
index yield an adamantine luster, meaning they display extraordinary brilliance and
shine.
Resinous - This
is the luster of many yellow, dark orange, or brown minerals with moderately high refractive indices - honey like, but not
necessarily the same color.
Silky - A silky
luster is the result of a mineral having a fine fibrous structure. Minerals with a silky
luster have optical properties similar to silk cloth.
Pearly - Describes
a luster similar to the inside of a mollusk shell or shirt button. Many micas have a pearly luster, and
some minerals
with a pearly luster have an iridescent
hue. Some minerals may exhibit a pearly luster on cleaved crystal surfaces parallel and below
the reflecting surface of a mineral.
Greasy - Luster
of a mineral that appears as if it were coated with grease.
Pitchy - Minerals
with a tar-like appearence have a pitchy luster. Minerals with a pitchy luster are usually
radioactive and have gone through the
process of metamiction.
Waxy - A waxy
luster describes a mineral that appears as if it were coated with a layer wax.
Dull - This
luster defines minerals with poor reflective qualities, much like unglazed porcelain. Most
minerals with a dull luster have a rough or porous surface.
Every mineral has a characteristic luster, but some minerals may have a different luster
on different specimens. There is no scientific method to determine luster. Often,
determining the luster of a particular specimen is personal; to some it may appear as one
type of luster, and to others as a different type.
How to use luster as an identification
mark
It is in the eyes of the viewer to determine what luster a
particular specimen exhibits. Luster is only a useful form of mineral identification when
the specimen in question displays a unique luster, such as waxy, greasy,
pearly, etc. Specimens with a vitreous luster cannot be distinguished from one another,
nor can minerals with a metallic luster. Luster is usually just noted as a mineral
property, and is not commonly usually used to help identify a mineral.
How to test using luster
Observe the specimen in well lit conditions where its
luster is visible. The surface being viewed should not be tarnished, unclean, discolored,
or coated. Some minerals exhibit a pearly luster on cleaved surfaces, so it is a good idea to
check for luster on uncleaved portions of the crystal.
