The Tyranitarite is not available in Pokémon Y as it's Version Exclusive in X. It is located in the 2nd Gym which is in Cyllage Town.
You will need to have completed the game, defeated the Elite 4, battled your rival at Kiloude City and updated your Mega Ring. Then go to the Gym between 8-9 PM and you should see it sparkling somewhere in the east of the Gym hidden behind a wall.
This video should help.
