Purchase a large piece of stone, about 18 inches by 3 feet, in the finish material desired. Different finishes allow for different degrees of intricacies. Sandstone, marble and soap stone are considered good for carvings and inlay work, so if you will be creating a stone garden chair with fancy carving creations, choose one of these softer stones.
Begin to chisel out a large section of your stone, leaving the bottom portion fully intact. You will use a large solid metal chisel, and a lightweight mallet to remove the stone. Your chair will have a solid stone seat, about a foot from the ground, with an attached back. You will slowly and carefully chisel away the area of stone above the seat and in front of the back. Because the chair will be chiseled from one piece of stone, your stone garden chair will not contain joints.
Sand your stone garden chair using an electric water sander. While you are sanding you must use plenty of water. The water keeps the heat from the friction, and the stone dust, at a minimum. Continue sanding your stone garden chair until you have your desired texture. You might want to keep some of the stone rough and other parts smooth, or smooth out the entire chair. It will depend upon the look you're creating.
Lift your chair carefully when moving it to your garden. If you did not carve your stone garden chair in the garden, it is going to need moving. Some stone chairs weigh in at about 200 lbs.. Whether you relocate your chair using a wagon or dolly, you will need to use the utmost precaution. Not only do you want to prevent your chair from cracking, you also don't want to break your own body.
Add a nice floral patterned seat cushion to the seat of your stone garden chair, for aesthetic purposes, and of course, for comfort. Stone can be pretty tender on the backside if sitting for any length of time. Using small tipped chisels, carve a few simple scroll patterns in the back of your chair, or if you are the artistic type, you might want to carve some actual pictures. Be creative. It is your stone garden chair masterpiece.