I used to be huge into clam keeping, but the rarest clam I have ever seen, in person, was a true blue Squamosa in Orlando FL at a shop called Sea in the City. It was extremely healthy and absolutely gorgeous.
I have seen other clams that could be considered rare, but the blue Squamosa is the most memorable to me.
The rarest clam I have ever owned was a hybrid Maxima/Crocea clam. It wasn't gorgeous, but it was definitely interesting!
I know what clam you are talking about. Sea In The City always gets nice clams in. I go there about once a week. Even LiveAquaria says the all blue Squamosa is very rare to find and will hold a much larger price tag on it.