Lustre | The Reflection or Scattering of Light
Lustre is a description of the way that light interacts with the surface of a crystal rock or mineral. Lustre is the reflection or scattering of light from the surface of the stone. The words used to describe different types of lustre include;
Rocks minerals and gemstones with a vitreous lustre reflect light in a similar way to glass. Vitreous is one of the most common types of lustre and is often present in stones that are translucent to transparent. For a stone to exhibit a vitreous lustre it needs to have a relatively smooth surface and high reflectivity. Many gemstones exhibit this type of lustre particularly after having been polished.
Lustre is often confused for brilliance but the two properties are not the same. Lustre refers to the way light is reflected off the surface of a rock mineral or gemstone whereas brilliance is the way it reflects from within a gemstone. Brilliance can only be seen in gemstones that are highly translucent to transparent and can vary depending on the way the gemstone has been cut. Gemstones tend to be faceted to maximise brilliance. Polishing will almost always improve lustre.
There are two broad types of lustre, metallic and non-metallic and all materials fall into one category or the other. Metals have a metallic lustre whilst most other rocks and minerals are non-metallic. Two exceptions are the minerals hematite and pyrite.
The word lustre comes from the Latin 'lux' meaning 'light' and there are many terms used to describe it. In addition to those mentioned above others include pearly, silky, resinous, earthy and adamantine.