The Formation of a Natural Pearl

[Caitlin]

I am referring solely to bivalve mollusks in this post.

So many sites use something similar to the following description for the formation of natural pearls:

Pearls form inside oysters, mussels, and clams (molluskan bivalves) when a small object becomes trapped inside the shell of the mollusk. Contrary to popular opinion, it is not sand that finds its way inside the shell but a small piece of clay, mud, worm, small crab, or parasite. When a foreign object enters the shell, as a self defense mechanism, the mollusk attempts to isolate the foreign object by secreting nacre (also known as mother of pearl).

However, here is Elisabeth Strack's (2006) definition: I think Strack's definition of how a pearl is formed is the best yet. pp 114, & 116-117.

She gives credit to Friederich Alverdes who in 1912-13 discovered the following:

Pearls are formed by the intrusion of epithelial cells into the mantle. The cause of that intrusion may be anything from a fish biting the mantle, thus carrying epithelial cells into the mantle to worms carrying epithelial cells into the mantle. In short, anything that puts epithelial cells into the mantle will form a pearl sack.

So the statement that reads

When a foreign object enters the shell

would better read "when a foreign object enters the mantle tissue carrying a few epithelial cells with it, a pearl sack is formed"- There may or may not be anything foreign lodged in the tissue for this to happen, but it is imperative that the epithelial cells be present in the mantle to form a pearl sack.

The description given above may fit natural mabe pearls which are formed against the shell, but not for free standing pearls.

 

[J Marcus]

Truly fascinating! It reinforces the impression I have formed about pearl formation from my admittedly limited study of abalone pearls. However, I have had the "advantage" of having blundered my way through about 7 (Yes! Sniffle, seven, whimper. . .) of these pearls with either diamond rock saw or lapidary grinder. They were all completely enclosed within the shell and there was no way of knowing of their existence before I made the cuts. The only thing at the center of these was a miniscule nub of what seemed to be--nacre. Still, there are quite a good number of pearls that do form around foreign objects that get into them. I once dug up a large (6 or 7 inches) horse clam locally that had built a few layers of nacre over about a half-handful of sand and gravel that had somehow gotten into it's shell--about as homely as a tridacna concretion. . . Perhaps it was the result of a bad case of hiccups??? In any case it apparently resulted in an upset stomach--for the clam, that is--it was still delicious for us humans!

 

[Caitlin]

I don't think Strack's definition of pearl formation includes abalone. If something bored into the abalone mantle, the abalone would bleed to death. Abalones are hemophiliacs.

Abalone natural mabes have been said to be made by shell boring critters. It also seems to be the case that the mabe made by such a happening will eventually be expelled through the outside of the shell. I think I saw this in Streeter, in reference to abalones. He also said the shell wears on the outside or it would be very, very thick, thus the appearance of the pearl moving through the shell.

Examples of the abalone mabe moving to the outside have been shown in various stages in this forum. Are you one of the ones who posted them?

 

[J Marcus]

Actually, Caitlin, I was not referring to blister pearls but rather to pearls that begin as solid, free pearls, formed in the mantle, that have come loose from the mantle, then apparently held in place against the shell while nacre or conchiolin was being secreted, thus cementing the pearl to the inside of the shell. Eventually, I have found, some of these pearls get covered by succesive layers of nacre--sometimes even to the point that they become wholly incorporated in the shell so that some of them are completely hidden with no remaining external evidence of their existence. ...until one cuts into them, that is. . . These natural, whole, solid pearls have no apparent relationship to any parasites like almost all of the blister pearls do. I realize that abalones are hemophiliacs, so I don't know how the natural solid pearls in abalones get started. However, my conjecture is that very tiny wounds in them probably are able to heal before they bleed out, if indeed they begin that way. I have both photographs and X-rays of this phenomenon and I will post them here as soon as I am able to find and get them together. Yes, I have posted on both the subject of blister pearls migrating to the outside and also a bit on solid abalone pearls. I think that those posts are in the Natural Pearls forum.

 

[jshepherd]

Sorry guys. This is a necessary edit. We will be able to continue the conversation at a later date.

 

[barbie biggs]

03:04 AM. One wonders if The Rock ever sleeps.