Diving photographer Kurt Amsler gets a lesson in pearl farming

The great Poe Rava, or black pearl, comes from Pinctada margaritifera, a black-lipped oyster traditionally fished off Tahiti, using mortally dangerous methods. But in 1962, diving fishermen motivated by a pearl jewellery boom started the production of cultured Poe Rava pearls. Since then, the lagoon pearl farms of the Gambier Islands, in the Tuamotu Archipelago, have become the treasure house of French Polynesia, supplying black pearls to world markets.
Diving at one of these farms, I sank into murky lagoon water to see the most amazing sight - hundreds of oysters, suspended on stakes hung side-by-side from the surface, to a depth of about 20m. The diving pearl farmers tend their crops with loving care (left), cleaning the shells regularly and checking that the suspension rigs are placed in the best currents.
The oysters are left to grow for four or five years before they are brought to the surface so that a foreign body, such as a grain of sand, can be grafted into them. Back down they go for several more years, during which time the oyster instinctively surrounds the injected foreign body with nacre, so building up a beautiful pearl. Of two pearls produced in an oyster's life cycle, the first will be 8-9mm wide, the second bigger at 10-12mm. The record is 21mm.
Poe Rava pearls are rarely fully black. Their thick layer of nacre - some 1-3mm - is more usually a mix of hues, and just how desirable they are depends on colours achieved. The most desirable are peacock green, aubergine purple, ocean blue, various greys and iridescent colours.