Several years back, a ceremonial tabbard was made with a tree in full foliage on the front and the Sigil on the back with its wreath and stavewood matching the species of the tree. Textile artists among us could experiment with making vestments that follow this pattern, using different types of trees for different ceremonial functions or occasions. As far as jewelry is concerned, currently the Druid Sigil is available in the form of silver rings, thanks to well-known Pagan jewelers Fred and Jill Buck. About a dozen of us are wearing these now, and they do look rather nice. If you're interested, they can be ordered directly from me.
Nobody knows for sure.
It first became associated with Druidism in modern times by the founder of the Reformed Druids of North America, David Fisher, twenty years ago. He claimed that it was a symbol of Druidism in general and the Earth-Mother in particular. Others think he may have gotten it from a Mesopagan Druid organization to which he may have belonged. Regardless of its historical origins, I think that it is a quintessentially female symbol, in both the Freudian and the Jungian senses, and is thus psychologically powerful. For political and metaphysical reasons, I think it's important for members of a religion that many folks assume is male dominated to have a constant reminder of the eternal power of the female forces of Nature.