The Blackstone is a sacred object for shrews. Leaders of Guosssom transmitted it to their sons.
It is this stone which makes a shrew Log-a-Log, and is the symbol of their authority. It is sometimes seen carried by paw or it is worn around the neck. The Blackstone, or some form of it, is used also in the original book Redwall. In that book, the one who holds the stone is the only one who can speak during a meeting.
Nordo, the son of the Guosssom Log-a-Log lost the stone while he was playing with it. But one day, Log-a-Log went to the island on the Great Inland Sea. When he was sleeping in a wood of the island, he saw Urthwyte with the Blackstone. Thinking it was a ghost, he ran away. He asked Mara to recover it because she was a badger, like Urthwyte. When they arrived on the island, they discovered Urthwyte, Loambudd and Ashnin. Mara told them what happened at Salamandastron and Urthwyte gave her the stone.