How big is the largest pearl oyster?

The first time you lift a mature Pinctada maxima from a panel net, the surprise is the weight. It is a dinner-plate shell, dripping and crusted with sea growth, with a slick of silver or gold shining at the lip. Reaching up to around 30 centimetres across, it dwarfs every other pearl oyster in the trade.

Everything fine about a South Sea pearl starts with this animal. A bigger oyster accepts a bigger nucleus and lays down nacre over a larger surface, which is why South Sea pearls reach sizes the smaller oysters never could.

How the lip sets the colour

There are two varieties, and you can read the pearl from the shell. The silver-lipped Pinctada maxima grows white to silver pearls. The gold-lipped variety grows champagne to deep golden ones. Both colours are entirely natural, never dyed, set by the same pigment that lines the shell.

Australia produces most of the world's white South Sea pearls, while the Philippines and Indonesia are the main sources of the golden ones. When you see a true golden South Sea pearl, you are looking at the gold-lipped lip made round.

Where South Sea pearls come from

South Sea pearls grow in the warm, clean tropical waters of Australia, Indonesia, the Philippines and Myanmar. The water has to be near pristine, because Pinctada maxima is a sensitive filter feeder that will not produce gem nacre in poor conditions. That fragility is part of why the pearls stay rare.

It also explains why you cannot farm these oysters just anywhere. A single polluted season can ruin a harvest, so the farms cluster in a handful of remote, protected bays. When you buy a South Sea pearl, you are buying water quality as much as craft.

The oyster on the 1000-peso note

Here is the detail that surprises people. If you have ever held a Philippine 1000-peso bill, you have seen this oyster. Pinctada maxima and its pearl have featured on the back of the note since 2010, beside the Tubbataha Reefs. A whole country put its largest pearl oyster on its money, which tells you how much the South Sea pearl means there.

Pinctada maxima and its cousins

It belongs to the genus Pinctada, the saltwater pearl oysters. Its dark cousin Pinctada margaritifera grows the Tahitian pearl, and the smaller Pinctada fucata grows the Akoya. Read the black-lipped oyster guide for the dark side of the family. Pinctada maxima is simply the giant of the three.