What do I use to adhere decorative stones into an interior doorway threshold?

So you'll have an idea.... the living room (carpeting) abuts poured concrete slab (slab was poured atop old metal framework from previously existing slider door). I have removed the concrete slab and exposed the old metal. I would like to fill this with decorative stones, but don't know what to use to keep them in place....an acrylic-based adhesive? How am i going to make sure the surface layer is smooth? I saw (on TV) it's been done, but missed how to do it! The door-less threshold extents into the now-enclosed porch.