If held by a Steelix, the Steelixite allows it to Mega Evolve into Mega Steelix 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 a Steelixite from Steelix.
Outside of battle
If held by a Steelix, the Steelixite allows it to Mega Evolve into Mega Steelix when it performs a Spectacular Talent during a Pokémon Contest Spectacular if its Trainer has a Key Stone. Steelix will remain a Mega Steelix until the Talent Round ends.
A Steelixite worn by Brock's Steelix appeared in When Regions Collide!, where Brock used it to Mega Evolve Steelix during his battle with Kiawe. The Steelixite didn't appear equipped on Steelix, but rather, it appeared on its head right before Mega Evolving.