If you’ve watched our jewellery presentations for any time at all, you’ve probably heard the word ‘lustre’, but what does this common term mean? Lustre describes the way light interacts with the surface of an object, particularly gemstones and minerals, and is derived from the Latin word ‘lux’ meaning ‘light’. The critical distinction here is the word ‘surface’, as the light that enters into the gem and then bounces back out is known as ‘brilliance’ instead. If light enters the gem and is split into a rainbow of colours, it is known as ‘fire’ or ‘dispersion’.
Imagine a completely black, opaque gemstone such as Black Onyx or Black Spinel. No light is entering the gem, so all the light reflecting back as you move it around in the light is lustre. Lustre is generally stronger from harder and denser gemstones such as Diamond and Zircon, but all gems have lustre, whether they be transparent, translucent or opaque. Not all gemstones have brilliance or fire.
This covers lustre at its most basic level, but the quality of the lustre can be further sub-defined into categories that describe the type of lustre that a gem can display. The boundaries between these types of lustre are not rigidly defined. Furthermore, the same gemstone can show different types of lustre depending on their origin, formation conditions and any number of other variables, so it is certainly not an exact science.
Diamond and Zircon have a gorgeous, sparkling lustre known as an adamantine lustre, which has also been described as ‘mirror-like’. Adamantine literally means ‘Diamond-like’. Ruby and Sapphire are sometimes described as having ‘sub-adamantine’ lustre in rare cases, though many are classed as vitreous.
The most common lustre type found in faceted transparent gemstones is similar to that seen from a pane of glass and is known as vitreous lustre. Emerald, Aquamarine, Spinel, Topaz and Tourmaline are amongst those that have vitreous lustre. ‘Vitrum’ is the Latin word for ‘glass’, from where this term is derived.
Gemstones with a pearly lustre obviously have an appearance similar to that of organic Pearl and Mother of Pearl. Their appearance is normally due to microscopic layers within the gem from which light reflects in an unorthodox yet beautiful manner. Charoite is one such example.
Silky & Fibrous Lustre
Gems with a silky lustre have very fine fibres (just like silk) which are arranged parallel to each other. Malachite and Sillimanite are both said to have silky lustre. A fibrous lustre is similar to a silky lustre but has a coarser texture. Tiger’s Eye has a fibrous lustre.
Gems with a greasy lustre are those whose surface reflection is similar to that of grease. This is caused by a mass of microscopic inclusions within. Peridot, Alexandrite, some Opals and some Garnets are said to have a greasy lustre. They can even feel greasy to the touch.
Resinous lustre is similar to the appearance of resin or smooth plastic. Amber is one of the most well-known gems that is said to have a resinous lustre, which is unsurprising given that it is considered a plant resin. Others include Sphalerite, Sphene and Vesuvianite.
As the name suggests, gems with a waxy lustre have a surface appearance similar to that of wax. Jade, Chalcedony and Turquoise have a waxy lustre. Some Opals are also considered to have waxy lustre, though this can sometimes be polished to a resinous finish.
The word dull may not have the most positive connotations, but in gemology terms, it simply means that the gemstone in questions reflects very little light. This is often because the stone has a coarse surface, scattering light off in all directions. Rhodonite is one such example.
Metallic lustre is similar to that of polished metal and is not often seen in the gemstone world, more frequently being used to describe the metalwork of the jewellery itself. Pyrite, Marcasite and Haematite are notable gem exceptions.
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