I'm quite certain this was originally the top of a glass bottle stopper that broke off evenly due to a flaw in the manufacturing. Kind of like when you break a pair of pliers on a metal fault... (frustrating) but in this case it created a beautiful perfectly round and exquisitely frosted gem. The slightly off clear colour hints at an even older origin than most sea glass.
As in all my jewellery the sea star and wire is Sterling and the little crystal is Swarovski. This is the only piece like this in my 20 years of collecting that I have found.