Stone Mania

Lustre, which can also be spelt luster, is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word traces its origins back to the Latin word "lux", meaning "light" and generally it still implies radiance, gloss or brilliance.

Broadly speaking there are two types of luster, metallic and nonmetallic.  Minerals with a metallic luster include those with an untarnished surface such as gold, steel or copper while those with a nonmetallic luster tend to have a lighter colour and display some degree of transparency or translucency although this may be minimal and only be visible on a very thin edge. 

There are a number of terms which describe nonmetallic lustres and there are no strict boundaries so the same mineral may well be described differently.  Some of the better known terms are:

Resinous - These minerals are transparent or translucent.  Amber is a resinous, it has the appearance of resin, chewing gum or (smooth surfaced) plastic.  It is a form of fossilized resin.

Dull - Although not a particularly enticing word especially when referring to a gemstone, a mineral with a dull lustre is non reflective and has the appearance of being very smooth.  An example would be meteorite. 

Vitreous - Calcite, Quartz, Topaz, Beryl, Tourmaline and Fluorite are all vitreous minerals.  This means they have a similar appearance to that of glass and this type of lustre is one of the most commonly seen and occurs in transparent and many translucent minerals.

Waxy - Unpolished Chalcedony and Jade both have a waxy lustre.  A waxy lustre simply means a lustre which resembles wax.

Chatoyancy - Chatoyant minerals display bands or markings which appear to move as the specimen is moved and catches the light.  Such minerals are composed of parallel fibers (or contain fibrous voids or inclusions), which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light.  The most famous example is Tigers Eye but there are also plenty of other gemstones which can be chatoyant.

Schiller - From the German for "twinkle", this term is used to describe the metallic iridescence originating from below the surface of a gemstone that occurs as light is reflected between layers of minerals.  It is most commonly seen in Moonstone and Labradorite and is also known as labradorescence.

 

 

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