The Gemstones are items found inside the Barkhang Monastery in Tomb Raider II. Lara will find two such gems in the level and use them to open doors. They are located behind a breakable window in a room underneath a courtyard with several golden trapdoors.
The Gemstones are inserted to special golden receptacles. First one is near the courtyard with golden trapdoors, between two golden animal statues. Placing the other one of the two Gemstones here opens a star-shaped door, which makes it possible to get one of the five Prayer Wheels needed in the level.
The second one is placed on a receptacle located in an alcove high on the wall of the main hall of the monastery, where the Buddha statue is. To get there, Lara must jump along the top of the statue. When the Gemstone is placed there, a trapdoor opens in the room under the statue.