Could someone please tell me what the use of the 'Rogue Stone' is?

you can sell 'em for cash or use 1 to enter one of the toughest fight in the game:

**SPOILER**
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Its the house opposite the temple in the bridge district; you'll need a rouge stone to enter the building. Good luck!

SPOILER
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In the bridge district there is a mysterious door which opens into a void which you cannot enter. If you have the stone, however, you will pass through into an area with a very tough party, comprising of a lich, beholder and a vampire amongst others. Although the spoils are good, don't attempt it unless you are sure of victory as you can only return by killing them.

what do you get for killing them?

A staff of the Magi.... i cant remeber stats exactly but it give you permenant invisibility & protection from evil among other things....

in other words the most powerful staff you can get in the game ... =)