Using organic elements in weddings is very popular right now. Earthy tones are often surrounded by branches, moss, and river rocks. I came across SJEngraving, a local Bay area vendor on Etsy, that had a very unique product. They will individually laser engrave river rocks.
I can imagine these being used as escort cards. They could engrave the table number right on the rock. Or, if you want to be able to have more flexibility in making table changes, you could put the rocks in small bags, using a tag to mark the table number. 
