The helmet worn by Darth Vader was constructed using an alloy of obsidian.
Concerning Obsidian: Obsidian is a material of note. The tower known as Fortress Vader, found on Mustafar, was built using obsidian. Furthermore, Vader's helmet was crafted from an alloy consisting of obsidian and durasteel. The walls of the Mirror cave cavern were also composed of obsidian.
At Dok-Ondar's Den of Antiquities, located in Black Spire Outpost on the planet Batuu, a selection of kyber crystals were available for purchase, housed in small containers. On rare occasions, instead of the expected red crystal, a fragment of obsidian could be discovered within a capsule. These distinctive obsidian fragments were extracted from shrines dedicated to the dark side and were imbued with the power of the dark side of the Force. Consequently, these fragments acquired characteristics akin to red kyber crystals, and when incorporated into a lightsaber, they would generate a red-colored blade.
The planet known as Oba Diah contained obsidian. Mok Shaiz's majordomo expressed a desire to witness the magnificent obsidian cliffs of Oba Diah personally, in an effort to impress the Pykes who were poised to launch an assault on Tatooine.