Real World Properties

Also called moonstone. The Romans believed hecatolite was made of solidified moon rays. It was often associated with Greek and Roman moon gods and goddesses. Moonstone has a pearly, opalescent luster caused by the many layers of feldspar that make up its structure. This bluish white shimmering effect is called adularesence.

Teen Wolf Mythology

In Teen Wolf, this stone "scatters moonlight," which means that any werewolf encased in hecatolite will not have felt the effects of the full moon. Peter equates this to starving lions to make them more vicious. 

Of course, any solid, opaque object scatters all light. That's the definition of opaque. What they might mean to say is that it also scatters whatever magic the moonlight carries with it that werewolves need, which can normally pass through other opaque objects.