What is the name of the gemstone that glows over a pokemon?

The gemstone that glows over a Pokemon is called a Mega Stone. Mega Stones are special stones that allow certain Pokemon to undergo a temporary transformation called Mega Evolution, resulting in a more powerful form with boosted stats and sometimes a different appearance. Each Mega Stone is specific to a particular Pokemon, and they are typically found in the wild or obtained as a reward for completing specific in-game tasks. Mega Evolution was introduced in the 6th generation of Pokemon games, starting with Pokemon X and Y.