THEORY TIME
So we saw that there are three meteorites
This is the Meteorite we know and love
This appears to be the meteor shard
What are these? Oddly shaped megastones?
Now as far as I'm concerned, all the other mega stones are smooth and round. We also know that Rayquaza's mega evolution is unique. It seems likely that this is Rayquaza's mega stone.
All 3 meteorites (not counting the shard), reference the same location. You obtain them at Mt Chimney, but the way they are worded implies that it's not 3 meteorites, but one meteorite that you obtain and slowly break down to reveal the gem inside.
Meteorite - A Meteorite that you got at Mt. Chimney. It gives off a faint glow.
Meteorite - A Meteorite that you got at Mt. Chimney. It
continues to give off a faint glow and is slightly warm to the touch.
Meteorite - A Meteorite that you got at Mt. Chimney.
Some sort of pattern has risen to the surface, and it’s shining with seven colors of light.
The trailer and official site mentioned that Rayquaza mega evolves through the influence of a Mega Stone of some sort.