So Iridescent Gem as of now is simply here so developers don't get asked stupid questions "why the gem on the Staff is white but when I deconstruct it it's yellow?". Maybe it could be used as a way of making new statues- the petrified creatures. With iridescent gem, ice staff and moonrocks you could craft a "Patrifing Staff" you could use it on some creatures such as (obviously) werepigs, hounds, maybe also rabbits, birds, spiders, frogs, bats. Big statues (werepig, hound) you'd have to carry just like marble pieces and small statues you could just carry in your inventory.
This idea would anwser the question "what is iridescent gem used for" and also create another way to decorate your base.
Or you can just make that a stronger version of ice Staff- you can turn every creature into stone, they'll break out of it but it takes longer then getting out of ice (yes just like that book in forge)