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Lustre (also  spelt  luster)  refers  to  the  surface reflection of light off an object and is a  description often used in gemmology to describe  the way light interacts with the surface of the  gem. Black Diamonds for example, don’t display  any dispersion or brilliance, but demonstrate as  much lustre as a white Diamond. 

In  fact  Black  Diamonds  and  Marcasite  often  out sparkle and out dazzle colourless Diamonds  because  their  lustre  can  be  greater  than  the internal  brilliance  and  dispersion  of  white  Diamonds.

Opaque  gems  are  normally  cabochon  cut  to maximise  their  lustre,  while  gems  that  are  transparent to translucent are often step cut or  brilliant cut in order to take advantage of their  internal brilliance and their ability to disperse  light. That said, step cut and brilliant cut gems  still  retain  the  ability  to  show  lustre  off  their  crown and table facets.

The  harder  the  gemstone  and  the  greater  its  density, the better the lustre tends to be. Diamonds  and  Zircons,  for  example,  have  a  gorgeous,  sparkling lustre known to gemmologists as an  adamantine lustre.

The most common lustre in faceted transparent gemstones,  is  similar  to  that  seen  in  a  pane  of  glass  and  is  known  as  a  “vitreous  lustre”.Aquamarine,  Spinel,  Topaz,  Emerald  and  Tourmalines are amongst those that are said to  have a vitreous lustre. 

Gems  with  a  greasy  lustre  are  those  whose  surface reflection is similar to that of grease. This  is  normally  caused  by  a  mass  of  microscopic  inclusions   within   the   mineral.   Peridot,  Alexandrite, Opals and some Garnets are said to  have a greasy lustre.

Metallic lustre is similar to that of a polished  metal or that of a mirror; Pyrite and Marcasite  are classic examples.

Gemstones with a pearly lustre obviously have an appearance similar to that of an organic Pearl. Their  appearance  is  normally  due  to  layers  within the gem, from which light reflects in an  unorthodox,  yet  beautiful  manner.  Opals  have  a  pearly  lustre,  which  is  often  referred  to  as  Opalescence.

Resinous lustre is similar to the appearance of  resin or chewing gum. Amber is one of the most  well-known gems that is said to have a resinous  lustre. Others include Titanite and Vesuvianite.

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.

As the name suggests, gems with a waxy lustre  have an appearance similar to that of wax. Jade  and Turquoise have a waxy lustre.

To really achieve the most enjoyment possible  out of your gem collection, it helps if you can  truly understand the difference between lustre,  brilliance and fire. Try reading up on all three of these  topics and then take out your gem  collection and try and identify all three of these  different visual effects. Once you can do this,  you  will  find  that  your  appreciation  of  your collection will grow immensely.

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Sparkling lustre coming from this Blue

and White Diamond pendant.