Maybe you could look at the construction requirements of either the glass or the crystal golem--though that's typically more wizard stuff...
Or you could go for animate objects, though that spell is marked for bards and clerics.
Or you could go for the feat Craft Construct.
Ruyan.
They could befriend a Crysmal, which is an outsider, but otherwise is literally a crystalline scorpion that eats gems for breakfast.
How would they house-break the new crysmal so it doesn't misbehave with the party's gems (and NPCs' gems, and random treasure hoards)?
That could work, but Planar Binding is extremely difficult without a Magic Circle vs. "alignment" spell, which won't work on a Crysmal, as they're Neutral. It might be easier to use the Planar Ally spell, since there's no binding required, though if you want to hand waive some of the details as the GM, that's up to you.
I think Planar Binding a group mascot is a cool idea!
@ Fuzzy-Wuzzy: I think as long as they kept it well fed, they could probably keep it from eating the stuff they don't want it to - it has an Int of 6, so I don't think it would be too difficult. THat said, though, it could become pretty expensive to keep it well fed; good point...
Ooo, I know---they eat gems so they can reproduce, so what you need to do is neuter it! A sufficiently high Knowledge(planes) check ought to tell you which bits to cut off.
:X