We bought it at an lfs about 4-5 years ago. It has always been that color, but the store owner probably didn't realize its pretty rare. Luckily its pattern has not become blurry like a lot clams do as they grow larger. I actually prefer its pattern over the rare spotted turquoise ones.
We bought it at about 6-7", currently the shell size is at least 12". It is a our pride and joy and a great mansion for our clowns. I figure one of these days we'll have setup a species tank just for it. ;]
When I buy new clams, I never put them in the system with this one, for fear of it catching some disease. It loves the light and is almost directly under our 400w 20k Radium.