This tile has 3x3mm magnets on the edges and a recess for a 6x3mm center magnet.

To achieve the effect pictured, I painted the bottom of the tile with a few colors, green, blue and variations of darker blacks to achieve some depth on some tiles.  Then I put polished stones in the bases and filled them with a clear 2 part epoxy.  I also gradually added ink (speedpaint will work too) to the epoxy as I went along to further add some depth / darker hues.