You should be able to find the gem before getting the quest.
Quest items can not be sold, and this one (like most) is unique enough you probably would not have dumped it in a stash somewhere.

The first post of the inventory file topic contains a link to the Lost Items topic (written for DD but it also works for BD) which describes how to use the downloaded inventory files to get the item.
Why did I type "demon bane's game" when I meant [i]gem[i]? <img src="/ubbthreads/images/graemlins/shame.gif" alt="" />