If the amethyst cave is the big one that fell to the ground, it should be placed right in front of the entrance door. According to feng shui, this is called gathering wealth.
If it is made of amethyst basin, that is, the mouth is facing the sky, it should be placed in the middle of the living room, which is where people gather.
If it is small, it can be placed anywhere, but it should be facing the door or window.