Trees and boulders can be used in anchors. Always strive to make an anchor redundant if possible (multiple trees, for example). But a >12 inch and well rooted tree can be used as a monolithic anchor depending on the circumstances.
Trees
Make sure the tree is at least 6-12 inches in diameter and has a secure root system. Have at least two slings around the tree for redundancy. When using the rope (such as fixing a rope to a tree in order to access top rope anchors) it can be permissible to use one strand but two is preferred.
Boulders
Very large boulders or blocks can be used in anchors if they are large and secure.
Thread
A thread is an anchor passed through a tunnel in the rock. This particular anchor has a cam backing up the thread.
Chockstone
A chockstone is a secure rock (already) wedged into a crack that can be slung with a sling or webbing and used as climbing protection. It should be evaluated for security before use!