I have the same question than Caitlin, I have one, and thought that Pen pearls were more porcelain-like texture than iridescent
These pearl are very nacreous, solid...heavy. Most pen-shells I've handled are "lighter" (protein is less heavy than nacre or calcite). Their color is kind of "bronze" (coppery-brown), possibly being fished out during summer. Many pearls come out in this color in August-September.
...clearly showing a transition to nacre towards the hinge of this Pinna Nobilis.