Gem of Oshtur

[Metallist]

Can anybody tell me what does it do?

I have no idea what is that item for. In case you don't remember which one is it, it is located in the entrance hall of the sanctum sanctorum, on a table to the left side of the house.

Thx in advance 

[fox456]

Yes, you can't talk to the Ancient One without it.  The gem is placed near the top of the mirror the first time you interact with it.

[Metallist]

That explains it

lol I always picked the gem without wondering why would I need it

[fox456]

Same here!  It took several times before I realized what was going on and why.