Rune Stones are needed to do Runic Magic. They are small, metallic squares that are stored in a Runebag and held between the fingers for casting (up to three of them at the same time). For each Word of Power, a Rune Stone exists (with exception of XEN and ZU), with the first letter of the word etched in runic on it. There are 24 different Rune Stones.
The Trinket Edit
In the original release of Ultima Underworld I, a pouch with six Rune Stones was included as a trinket. The Rune Stones were small, metallic squares, looking exactly like the ones in the game.