Blister Pearl is a pearly excrescence on the inside of the shell of a mollusk (like an oyster) that develops over a foreign body like granule of sand, mud or a parasite thereby creating a 'blister' bump or bumps.
 
Mother of Pearl is a  smooth shining iridescent substance forming the inner layer of the shell of some mollusks, especially oysters and abalones.
 
 
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* Due to being a natural material, size and shape will vary.