Mystic Plaque
From WikiRaider
The Mystic Plaques are items found in the level Floating Islands. Lara will find two such items, one located on a green island, another inside a small building that appears to be floating. The two plaques must be placed inside receptacles on opposite sides of a great gate.
In The Dragon's Lair, Lara finds another plaque artifact in the hands of one of Bartoli's cult members. This plaque is used to enter the lair of the dragon.