One of these finely cut and polished stones is normally indistinguishable from a jewel of the ordinary sort, although a detect magic will reveal its dweomer. 

When gazed through, the gem of seeing enables the user to detect all hidden, illusionary, invisible, astral, ethereal, or out of phase things within viewing range. 

Peering through the crystal is time consuming and tedious. 

The viewing range of the gem is 30" for a cursory scan if only large, obvious objects are being sought, 10" if small things are to be seen. 

It requires 1 round to scan a 200-ft square area in a cursory manner, 2 rounds to view a 100-ft square area in a careful way. 

There is a 5% chance each time the gem is used that the viewer will see an hallucination, see something that is not there, or possibly see through some real thing as if it were an illusion.