There is no regulation in place as regards grading of pearls. It is generally accepted that pearls are graded on the following 5 virtues –
The lustre of the pearl is its refraction of light through the depth of the nacre. Nacre is the natural secretion from the oyster that makes up the pearl. The thickness and even coating of the nacre creates the lustre of the pearl. Healthy oysters, which have been meticulously farmed and afforded the necessary time to lay down substantial layers of nacre, will produce the highest lustre. Quality pearls with high lustre will stand the test of time to be enjoyed by generations to come.
AAA – Very Good Lustre
AA – Good Lustre
A – Fair Lustre
-A – Poor Lustre
Size of the pearl is dictated according to the species and size of the oyster and size of the implant. The South Sea Oyster (pinctada maxima) produces by far the largest pearl. The oyster may produce between one and three pearls during its lifetime. With only some of the oysters living long enough to produce a third pearl (which is likely to the largest pearl that oyster will ever produce) these larger pearls are more highly valued for their rarity.
Pearls are generally measured using a milliguage and sometime by sieve, depending on the shape of the pearl. The measurement of the pearl is in millimetres across the width of the pearl. The weighing unit for pearls momme, an old Japanese measure of weight still used for all pearls. One momme is equal to 3.75 grams or 18.75cts.
The colour of the pearl varies according to the species of the oysters and its genetics.
The South Sea (pinctada maxima) Oyster has two varieties, the silver lipped and the gold lipped.
Silver- lipped Oyster produces the white pearl with shades ranging from white -silver to white-pink and cream-white. The most valued is the whitest of pearls with just a tinge of pink.
Gold lipped produces the golden pearl from a deep gold to a pale buttercup yellow. The most valued is the deep golden colour.
Tahitian (pinctada margaritifera) oyster produces pearls ranging in colour from various shades of grey, silver, charcoal, blues coppers and greens. The deeper colours displaying shades of blue and green are the most sought after.
Akoya (pinctada fucata martensii) oyster produces pearls, renowned for their beautiful lustre, boast beautiful white pink shades and similarly to the South Sea Pearl they can produce cream-white and silver-white colours. The white pink is the most highly prized, although a deep silver-white is also highly valued on account of its rarity.
Every pearl is unique and most pearls will have some small imperfection on their surface, pearls with less imperfections are more highly valued.
AAA – small imperfections on 5% or less of the surface
AA – small imperfections on 30% or less of the surface
A – Small imperfections on more than 30% of the surface
-A – larger imperfections on more than 30% of the surface
Shape of each pearl depends on how evenly or unevenly the oyster lays down the nacre during its cultivation period. Pearls are most often produced in round, near round, tear-drop and baroque shapes. Round pearls are the most highly valued.
R2 Near Round
Dr1 Smooth Drop
Dr2 Bell shaped, short squat drops
Dr3 Triangle (flat bottom), knob on the top or slightly asymmetrical drop shape