G-Metal

G-Metal is a metal (wow!) from the Headmasters portion of the Generation 1 continuity family.

Scorponok watches as shiny stuff is forged.

G-Metal (ジーメタル Jīmetaru) is a rare substance found on planet Master. Virtually indestructible, it was targeted by Scorponok to use in the creation of a shield that could repel the Master Sword.

Fiction

The Headmasters cartoon

Returning to planet Master after being forced off of Earth when his Transtector's weak spot was exposed, Scorponok took refuge beneath the surface of the planet, where his Transtector detected the presence of G-Metal ore. He planned to forge it into a shield to counter Fortress Maximus's Master Sword, but was aware that the Master Sword was even more durable than G-Metal alone, and abducted Wheelie in order to trade him for the secrets of the sword’s construction. The Master Sword Is in Danger!!

With this information, Scorponok perfected the Zarak Shield despite repeated attempts by the Autobots to prevent it. The Zarak Shield Turns the Tide During a battle with Fortress Maximus, the shield proved strong enough to hold off an attack by the Master Sword, allowing Scorponok to claim victory in the fight and return to Earth to begin his final plan. Operation: Destroy the Decepticons

Shattered Glass

When Optimus Prime was rebuilt into a new body following a defeat in the Mirtonian Quadrant, he was also fashioned a new sword crafted from mysterious G-Metal. Invasion