The fish that's also a pearl
Pearls are the result of a mollusc's reaction to irritants such as parasites that enter its shell.
Although model pearls are perfectly round and smooth, in reality they come in a huge number of shapes and sizes.
Watch Andreia Salvador, Curator of Marine Mollusca, take a look at one of the Museum's most extraordinary and precious pearls.
The pearl pearlfish
Among the millions of bivalve molluscs in the Museum's collections, one specimen stands out: a large oyster shell, donated to the Museum in 1886 and containing a peculiar pearl.
Rather than a spherical shape, the growth is spread over the shell's surface in the unmistakable shape of a fish.
Pearlfish are small, ocean-dwelling fish, often found in tropical waters. These translucent animals are known to swim inside clams and oysters to hide from predators.
However, for the individual that concealed itself in the oyster that is now in the Museum's collections, getting out of its hiding spot turned out to be as deadly as the predator it may have been fleeing.
By secreting layers of aragonite and conchiolin, the same substances that are in its calcium carbonate shell, the mollusc creates a material called nacre, commonly known as mother-of-pearl.
The oyster diffused this iridescent coating around - and effectively entombing - the fish.
Although naturally formed pearls are uncommon, they are typically created around small irritants such as invading parasites. This usually results in small, round pearls.
But the much larger fish was treated in the same way as any other irritant, and in turn became a unique pearl.
Pearls take a long time to form. Andreia believes that it would have taken about five years for the oyster to fully cover the pearlfish.