The internal life witnessed inside a gemstone.
There are three main terms used to describe the way light interacts with a gemstone: ‘lustre’, the surface reflection of light; ‘dispersion’, when light inside certain gemstones is split into the colours of the rainbow; and ‘brilliance’, the return of light to the eye from within the gemstone (hence why it is also known as ‘internal lustre’).
Everyone who collects gemstones will have their own views about what attracts them to certain coloured stones and not to others; I for one can’t always put my finger on why I love certain gems in my collection more than others.
However, a mixture of a gem’s colour, clarity and cut are normally the main reasons, even if the conscious mind does not necessarily realise it. These three elements also play a major role in the amount of brilliance seen in a gemstone.
When the colour of a gemstone is more vivid, its brilliance will normally not be as high. For example, you will see more brilliance in a Sky Blue Topaz than in a London Blue Topaz. The better the clarity, the fewer obstructions light has to negotiate in order to enter and exit the gem, the more likely you are to see a higher level of brilliance.
Most important of all is the quality cut of the gem: if the angles of the facets are poorly applied, the gem will not come to life!
Whilst a Diamond is cut to display a mixture of brilliance, dispersion and scintillation at their best, the key priority when shaping a coloured gemstone is how to best display the gem’s colour; and in the case of transparent gems how to also maximise the gem’s brilliance. The most important factor in the cut is the angle of the pavilion facets; in simple terms, the facets on the underside of the gem act like mirrors on which light is refracted. However, don’t expect all gemstones to have the same proportions and angles of that familiar picture of a Diamond, as different gemstones have different optimal angles for the return of light (known as a gemstone’s ‘critical angle’).
The best way to see brilliance is to hold the gem so that your light source is hitting the facets on the crown and then slowly move it from side to side in a rocking motion. What you should see are patches of light within the gem.If you can’t see any brilliance, then the gem is opaque (not transparent), heavily included, too dark in tone or poorly cut.
The colour seen by the eye from the lustre of a gemstone is known as its “key colour”, whilst the colour seen as a result of the gem’s brilliance is known as its key colour. This key colour is often assumed to be white, but on closer inspection, although lighter than the body colour of the gem, you will normally notice that it is in fact a coloured sparkle and not a white one.