I ordered a hideous teal colored cross with a wooden dove glued on the front off of ebay. I pried the dove off, chiseling away at the hot glue that was attractively visible around the edges of the dove. $10 well spent. Then I created a design on a paper outline with pre-cut stone mosaic pieces from an art supply store. I applied pre-mixed grout to the cross and pressed on the stone pieces.
I am happy with the result, but the grout is a little too white to feel rustic to me.