Mega Stones

Mega Stones are a Key Item in the Pokemon series, first introduced in Pokemon XY. Mega Stones can be used once in a battle to greatly increase the power of a Pokemon, as well as change their appearance, allowing for a "Mega Evolution--" an evolution that surpasses the final stage of Pokemon evolution but is only temporary.

Backstory
look up and see if theres any history on where the idea for mega stones came from

Mega Stones exist both in Sunverse and Moonverse, unlike their fictional (doubly fictional?) counterparts.

Appearance
Mega Stones look like large marbles, with a color scheme similar to the Pokemon they affect.

Purpose
A Mega Stone can be used in conjunction with a Keystone in order to cause a Pokemon to Mega Evolve.

History
Mega Stones were created when the Ultimate Weapon was used. Evolutionary stones around the world were exposed to radiation due to the blast from the canon, which eventually warped them into what they became currently.