The gem stone of Mani is the second reward from worshiping Mani of Machine. When used, it will transform one item in your inventory into another item, much like a rod of alchemy. It may only be used once every 24 hours.
In Elona+, this item is replaced with Mani's gem stone of satellite shooter.
The following items all act like the gem stone of Mani:
The item sprite ID for the Gemstone of Mani is 657 (Row 19, Column 30) using the item sprite zero-based position system, with a color modifier of 3.