Large, ancient stones, usually set upright. The formation at Stonehenge is an example of a megalithic structure. Many are found on ley lines. Some emit extensive magnetic fields.
Readers, please enjoy this guest blog post by Sandra Kynes, author of several books, including The Herb Gardener's Essential Guide, Plant Magic, and the new Witches' Encyclopedia of Magical Plants, among many others.
Although a few old books...