What is Lustre?

Lustre: The Reflection or Scattering of Light

The word ‘lustre’ describes how light interacts with the surface of a rock, mineral, or gemstone.

Lustre is the reflection or scattering of light, so it describes a stone’s appearance in reflected light.

There are two broad types of lustre, metallic and non-metallic, and all rocks, minerals, and gemstones fall into one category or the other.

Metals have a metallic lustre, while most other materials are non-metallic.

Some, such as hematite and pyrite, look metallic and reflect light like metals, but they are metal-bearing minerals, not metals.

Metal-bearing minerals are often ores, such as goethite, which is an important iron ore.

There are many types of lustre, but some of the most common are:

• Vitreous
• Dull
• Metallic
• Waxy
• Greasy

Stones with a vitreous lustre reflect light in a similar way to glass. Vitreous is one of the most common types of lustre.

For a rock, mineral, or gemstone to exhibit a vitreous lustre, it must have a relatively smooth surface and high reflectivity.

Moonstone is often described as having an opalescent lustre because of how it reflects light. The correct term for this optical phenomenon is adularescence.

Polishing a rock, mineral, or gemstone will usually improve its lustre, whether it's transparent, translucent, or opaque.

Stones with a dull lustre reflect minimal light, so their surfaces appear matte instead of 'glassy' or reflective.

A stone with a waxy lustre reflects light in a diffused manner, similar to how it looks on candle wax. Instead of producing a distinctive reflection like on a glassy surface, the light appears muted, giving the stone a smooth, slightly waxy appearance and texture.

A waxy lustre is often caused by a very fine or microcrystalline internal structure, which scatters light in multiple directions rather than reflecting it cleanly.

The word 'lustre' comes from the Latin word 'lux', meaning 'light'. In addition to those already mentioned, other types of lustre include pearly, silky, resinous, earthy, and adamantine.

What is Brilliance?

Brilliance in a gemstone refers to its ability to reflect and refract light in a way that makes it sparkle and shine. It can only be seen in translucent or transparent stones.

Lustre is often mistaken for brilliance, but they're not the same. Lustre refers to the way light reflects off the surface of a rock, mineral, or gemstone. Brilliance refers to the way light enters a faceted gemstone and is reflected back to our eyes. It's one of the main reasons for cutting facets into a stone.

The type of facet depends on the stone, its shape, and the desired effect. Each facet is cut to maximise the reflection of light, increasing the level of brilliance.

When light enters a gemstone, it bounces off internal facets before being reflected back out. The more effectively it does that, the more impressive the optical effect.

Brilliance describes the brightness and radiance of a gemstone as a result of light being reflected off facets. Gemstones with exceptional brilliance have a captivating and dazzling appearance.

The way a gemstone captures and reflects light maximises its beauty, which increases value.

Article Photos

The pyrite cubes and the quartz crystals: Courtesy of Stan Celestian.

The malachite, which has a dull lustre, is from our collection.

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Hematite, goethite: Courtesy of Stan Celestian.