What kind of gem am i looking to find for Haer'dalis? Where can it be found?

I believe it is in a trapped/locked altar/chest thingee in the mage tower thingee just back down the hall from where you originaly find Har.(where he is just standing like a zombie)..can't miss it

Think the chest is trapped.

It's an altar, and Nalia can't remove the trap. (Is it just poor Nalia, or any Thief? Never kept Yoshimo around long enough to know. Since this time I went & got Imoen right after cleaning out the Copper Coronet, maybe I'll find out).

Hrrrrrrrmmmmmm
Guys and girl, remember to put up a spoiler-alert.
This is "classified" information!
SPOILER
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First you go down to the temple slums. Then into the secret entrance in the north of the slums.
Go up down the stairs.
Go now to south, and then west and north.
There you will find he wizard (cant remember his name). Kill him, or work for him. Anyway what you choose you will end up with Haer'Dalis.
If you kill him, take his loot, and go back to the stairs. From there you go to right, and a bit in there you will find the gem. PS. Watch out for traps!

see ya!