Lustre (also spelled "luster") is hard to define. Pearls are not diamonds or sapphires – they don't refract light the way a gemstone does.
Luster for freshwater pearls is difficult to define. This is because pearls are different from other stones like diamonds, emeralds and sapphires. They don’t reflect and refract light the way these gem stones do.
Lustre is the sheen, the reflection, the glimmer that pearls have – or should have. It is the quality and the quantity of light shining on a pearl which is reflected from the service and just beneath the surface.
Luster is actually the gloss, glimmer and reflection of the pearls. It is the shinny surface of the pearl which is proportional to the amount of light that is reflected or refracted from the surface or just beneath the surface of the freshwater pearl
Otherwise perfect pearls, round and flawless, are less valuable if they are dull and boring.
If the pearls are dull and have no shine they would not have any value and importance. So the luster of pearls makes them to look lovely and beautiful. Compared to other pearls freshwater pearls have got a little less shine and luster but this doesn’t mean that they are not beautiful when compared to other pearls like saltwater pearls, Akoya pearls, south sea pearls and Tahitians pearls. They are equally stylish and elegant and are hugely popular amongst people who are fond of collecting pearls.