What's so special about this huge gemstone anyways?

[reason]

...

Lesson to derive from number of timing of responses: gemstones have a special place in human culture yet, far more so than merely rare gold, for all that they are about to become as cheap as soil and manufactured on demand at any size. Why would that be, do you think?

Some ideas on particular gemstones and troublesome men placing far too much weight on their existence here:

http://www.principiainfecta.com/archives/2005/12/thiefs_favor.php
http://www.principiainfecta.com/archives/2006/01/forbidden_gems.php

Reason
Principia Infecta

 

[Bryan898]

I had a huge gem be the focus for a cult in my campaign. The cult was spreading dogma in which it underminded the power of the true gods and spoke of the gods being cheap charlatans. It was led by Ur- Priests, and the initiates gazed into the gem for a minute during which time they experienced visions of the apocalypse and became faithful converts.

The gem was similiar to the palan'ti from Lord of the Rings, as Asmodeus was really controlling the visions, and the cultists through the gem. Anyone that stared into the gem was subject to his demand submission ability, Will save DC 37 or submit to Asmodeus for 10d10 days. His cult unknowingly empowered Asmodeus. IMC Asmodeus gains power from the "faithless", especially when they're led to turn away from faith.

Funny thing is... once the PCs got ahold of it, they promptly sold it without realizing waht happened. A few months later it appeared in the hands of one of the greatest warlords in the land, and the land is on the verge of total war since.