Coating
A pearl is formed by layered coatings of nacreous secretions over a nucleus. It takes thousands of crystalline layers to form a pearl. The quality of a pearl’s coating depends on the thickness, fineness and smoothness of these layers.
Lustre
This is the sheen that reflects light on the pearl’s surface. Lustre is a result of two types of light – that reflected by the pearl’s surface, and that reflected by crystal layers within the pearl. When these two types of light intersect, they reflect the depth of lustre and the pearl’s shimmer. The finer and thinner the layers are on a pearl, the greater its lustre.
Colour
The colour of a pearl is determined by the colour of its layers, which can range from red to yellow and even green, and the refraction of light through these layers. Black pearls can have coloured layers ranging from black, green, red, grey, blue, white and peacock green.
Size
Size is one of the most important criteria for evaluating black pearls. Cultivated for an average of two years, these pearls generally grow by 0.7 mm to 1 mm a year. Harvested pearls average 9–14 mm in size, though advances in cultivation technologies have produced even larger pearls.
Shape and flaws
Black pearls are classified in the following six shapes: round, drop, oval, circle, baroque and Keshi. As black pearls characteristically have warps and flaws, which occur during nacre (layer) formation, the smoother the black pearl, the rarer and more valuable it is. Smooth, drop-shaped pearls have become highly valued in Europe and the US.
Matching
Black pearl necklaces contain pearls matched in colour, shape and size. Because each black pearl is highly individual, it is extremely difficult to find pearls of the same quality to form a necklace. A necklace of black pearls is, therefore, valued comparatively higher than those of other pearls.