The Odin Sapphires (オーディーンサファイア) are items from Asgard. Each of the seven God Robes are adorned with one of these blue gems. Except for the Mizar Zeta Robe, all Odin Sapphires are located in the "buckle" of the God Robes.
All seven Odin Sapphires must be set in place on the crown of Odin's monumental statue in the Valhalla Palace in order to bring forth the Odin Robe and the legendary sword it contains: Balmung (バルムング). The death of a God Warrior automatically frees the Odin Sapphire from their God Robe, but Siegfried showed that it can also be voluntarily removed when he gave his to Seiya before facing Sorrento.
The gems do not confer any special power to either the Robes or the Warriors, as demonstrated by the wearer of the Alcor Zeta Robe, who was able to perform at the same level as the official God Warriors without one.