The Mobian Mega Crystal is a white, yet diminutive crystal, being no bigger than a thumb. However, it is quite powerful, and was designed to power the Subatomic Slow-go Beam Weapon, a weapon that slows down molecular motions. However, should the crystal be broke in half, it will result in the Subatomic Slow-go Beam Weapon having the opposite effect.
The Mobian Mega Crystal took nine years for Dr. Robotnik to make as a means of fueling his Subatomic Slow-go Beam Weapon. With the crystal powering the Slow-go Beam, Robotnik's henchbots, Scratch and Grounder, armed themselves with the weapon and managed to use it on Sonic so Robotnik could catch him. During a struggle for the Slow-go Beam between Tails, Scratch and Grounder however, the Mobian Mega Crystal was broken in half. Regardless, Scratch put the shattered crystal into the Slow-go Beam, unaware that by powering the Slow-go Beam with the shattered crystal, he had reverse the Slow-go Beam's effect. After Tails got a hold of the weapon, he used to make his allies fast enough to help him save Sonic and return Sonic to normal. Afterward, Sonic tossed the shattered Mobius Mega Crystal into a river to ensure that Robotnik could not find it again.