If held by an Abomasnow, the Abomasite allows it to Mega Evolve into Mega Abomasnow if its Trainer has a Key Stone. A Trainer can only Mega Evolve one Pokémon per battle (or only have one Mega-Evolved Pokémon at any given momentZA).
As a Mega Stone, it is unaffected by item-negating effects (such as Embargo and Klutz). Item-manipulating effects (such as Trick and Fling) cannot give or remove an Abomasite from Abomasnow.
Outside of battle
If held by an Abomasnow, the Abomasite allows it to Mega Evolve into Mega Abomasnow when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Abomasnow will remain a Mega Abomasnow until the Talent Round ends.
An Abomasite appeared in A Real Icebreaker!, worn by Wulfric's Abomasnow. The Frost Tree Pokémon wears its Mega Stone on its chest, revealing it only when it was about to Mega Evolve. Wulfric used it during Ash's rematch against him, in which it battled Greninja.