Lustre and surface appearance are the most important characteristics of pearl quality. Similar to the brilliance of diamonds, a lustrous pearl has more than just a shiny, reflective surface. It also has a glow from the light reflecting off the thousands of layers of nacre within.
Lustrous pearls are also iridescent; they not only reflect light, but they break it up into different colours. On white round pearls, this tends to be very subtle and a pinkish tone may result. On high lustre baroque pearls, there may be flashes of rainbow colours.
• Excellent lustre: very sharp reflections : bright, shiny, lively and clear (mirror like)
• Good lustre: bright reflections but not so sharp
• Fair lustre: weaker reflections, not so clear
• Poor lustre: dull, with faint or no reflections
High lustre is an indication of a healthy coating and is thus an assurance of durability. Very high lustre pearls have sharp, intense and almost mirror-like reflections, and there is a high contrast between their bright and dark areas. Very low lustre pearls are easy to spot. They look milky or chalky, and seem more like a white bead than a pearl. This is due to the low contrast between the light and dark areas of the pearl. There cannot be a good lustre on a thinly coated pearl.