Spirit-Gem

The Spirit-Gem was a mystic artifact that was metaphysically tied to the Meditation Realm. Nicol Bolas used to wear it between his horns.

Description
The Spirit-Gem was a smooth egg shaped stone, with a silken sheen. Its color was between gold and silver. Bolas used the Spirit-Gem as a channel to absorb the planeswalker's sparks that he had harvested through the Elderspell during the War of the Spark.

Found on the Meditation Plane after killing his brother Ugin, Bolas was unaware that it was made from a piece of Ugin's essence. Resurrected as a Spirit Dragon, Ugin used his connection with the Spirit-Gem to observe the events of the War without being on Ravnica. After the defeat of Bolas, Liliana Vess had picked up the Spirit-Gem before she planeswalked away.

The Spirit-Gem can reveal its possessor's “true, best self” and allows Ugin to communicate with its holder. In Caligo on Dominaria, Ugin briefly spoke through the Gem to Araithia Shokta and appeared to Liliana Vess who would have to make a choice on her path to redemption. Araithia would become visible to everyone, when she held the gem. Later, when Liliana held the Spirit-Gem in one hand and the Chain Veil in the other, the Spirit-Gem began to glow. As it disappeared, Liliana dropped the Veil and gave it up. The Gem's location is officially unknown, although it presumably returned to Ugin.

Trivia

 * The Spirit-Gem looked similar to, but wasn't, the Gem of Becoming.