When I gave the seminar on the Cathars, a Christian who was present, obviously disliking the Cathars’ lack of ascription of positive value to life in the physical world, suggested that the use of the Pearl as a Gnostic symbol might be meant to convey that something very valuable was created by the friction between idealistic spirituality or aspiration and life in the physical world.
Other people have also tried to see this in it, but I think this interpretation depends on modern understanding of the process of pearl formation, which is unlikely to have been understood until relatively recent centuries.
So you have to consider what the associations of the pearl might have been about 2,000 years ago, or in any fairly primitive community. In the natural state, a pearl is something very valuable that is found rarely and unpredictably in a minority of oysters, and pearls of insignificant size are a lot more common than the unusual large ones which are really valuable. Even seed pearls are not extremely common, and a person who was opening oysters all the time could go through a life-time without finding a pearl of real value.
Then you have to consider the difficulty and danger of obtaining them. These dangers exist even for modern divers with highly evolved diving suits and breathing apparatus. But a primitive diver has little to protect him from the various hazards which may await him under water.
There are ways in which a pearl diver provides quite a good allegory for a person trying to do something in the benighted (underwater) world. He is entirely dependent for continuing life-function on his air-supply from above, and cannot be cut off from it for too long. There is the risk of the bends, of being trapped underwater in various ways, and of being attacked by sharks, squids etc.
So it would appear that to the ancient world the pearl must have appeared as something of great value which was very rarely and unpredictably found, and which could only be obtained by diving into a difficult and dangerous situation.