Rocks, crystals, and minerals with a fatty, oily, or greasy luster have a surface that looks like they have an oily layer on their surface. Most also feel greasy (smooth or slippery), but don’t leave a residue on your hand when you touch them.
Luster is an optical property describing the light-reflecting character of crystals, rocks, or minerals, including gemstones. It tells us more about the quality/kind and intensity of light reflected or how they will appear in reflected light.
Learn more about the greasy or oily luster, including examples of some minerals with this shine. We will mention what causes them and a lot more.
What is a greasy luster?
Greasy luster describes some crystals, rocks, and minerals that appear as if they have grease, oil, or fat film on their surface when in reflected light. It is one of the kinds of non-metallic lusters, others being adamantine, resinous, pearly, waxy, vitreous, and dull or earthly. Of course, metallic is another kind of shine, too.
Minerals with a greasy or oily luster also tend to have a greasy feel. However, not all minerals, rocks, or crystals with a greasy feel have this shine too. For instance, graphite’s luster is metallic, submetallic to dull, barbertonite is waxy to pearly, and molybdenite is metallic, yet their feel is grease-like.
What causes this appearance in reflected light? This optical phenomenon occurs in minerals with microscopically rough surfaces. These surfaces will create the oily shine you see as they reflect some light.
Examples of minerals, rocks, or gemstones with an oily or greasy luster include carnallite, descloizite, mottramite, iowaite, fermorite, sarkinite, and tooeleite. Others, like sulfur, nepheline, serpentine, and milky quartz, have a greasy luster but will often show other kinds too.
Most greasy minerals and other luster kinds
Deciding which mineral has a greasy luster is subjective, with no clear boundaries distinguishing it from other kinds. Also, more than often, the same types of minerals may show more than one kind.
For instance, some hydrotalcite, cordierite, amblygonite, antigorite, pollucite sodalite, lawsonite, baddeleyite, danalite, chrysocolla, hurlbutite, zaratite, brockite or even nepheline may be greasy to the vitreous. Others like kleinite, becquerelite, kleinite, hedyphane, nasonite, and even diamonds are adamantine to oily.
Still, you will find resinous to greasy specimens like sulfur, realgar, fillowite, kasolite, or goyazite, while minium, litharge, massicot, and saponite (piotine or soapstone) are greasy to dull. Others may be greasy – waxy, submetallic, metallic, dull, or show several kinds of lusters.
That is not all. Some minerals, such as cassiterite, orpiment, libethenite, johnbaumite, and tazheranite, will only show a greasy luster on their fracture surfaces.
The point is you will always get the same mineral as having several kinds of lusters, with some sources listing different types. It is normal since this optical phenomenon is subjective.
References
- Anthony, J. W., Bideaux, R, A, Bladh, K. W., & Nichols, N. C., (Eds.) (2022). Handbook of mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/
- Nesse, W. D. (2018). Introduction to mineralogy (3rd ed.). Oxford University Press.
- Manutchehr-Danai, M. (2009). Dictionary of gems and gemology (3rd ed.). Springer.
- Rafferty, J. P. (Ed.). (2012). Minerals (1st ed.). Britannica Educational Pub. in association with Rosen Educational Services.