What Does Lustre Mean?
Lustre Reflection or Scattering of Light
The meaning of lustre when used in relation to rocks minerals and gemstones is the way light interacts with the surface of the stone.
Lustre is the reflection or scattering of light so it describes the stone’s appearance in reflected light.
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. It's often present in stones that are translucent or transparent. Stones with this type of lustre are often described as being 'glassy'.
Quartz crystal has a vitreous lustre. A polished ruby, amethyst or malachite gemstone can also have a vitreous lustre. Moonstone is often described as having an opalescent lustre becuase of the way light reflects off the surface of the stone.
For a stone to exhibit a vitreous lustre it must have a relatively smooth surface and high reflectivity.
Lustre is often confused for brilliance but the two are not the same. Lustre is the way light reflects off the surface of a stone. Brilliance is the way it reflects light from within a cut gemstone. It can only be seen in highly translucent to transparent gemstones.
One of the main reasons for faceting a gemstone is to maximise brilliance. The way a stone is cut can affect brilliance.
Polishing will almost always improve lustre whether a stone is transparent, translucent or opaque.
Rocks minerals and gemstones that exhibit a dull lustre absorb light instead of reflecting it.
There are two broad types of lustre, metallic and non-metallic. All crystals rocks and minerals fall into one category or the other.
Metals have a metallic lustre whilst most other rocks and minerals are non-metallic. Two exceptions are hematite and pyrite.
'Lustre' comes from the Latin 'lux' meaning 'light'. In addition to those already mentioned, other types of lustre include pearly, silky, resinous, earthy and adamantine.