Marble coasters make a wonderful gift! I made a set of four of these for a gift exchange. I stamped the central image (SU) first with black StazOn. Created a paper mask and then stamped the French Script around it in the same ink. I then "watercoloured" the image with watered down acrylic paint so that it would be permanent when dry. Did give the tiles a bit of a spray of matte acrylic once everything was dry. Added little cork feet, tied with a nice ribbon and I was done!! If you are needing some inspiration there are LOTS of coasters in the gallery on Splitcoast Stampers!
Great as last minute gifts. The marble tiles are available at Home Depot here and maybe other home improvement stores in your area. Definitely recommend Staz ON ink for crisp and clean lines if you are going to stamp and colour your images. I have used Memories Inks for images I wanted to be a certain colour like green leaves, etc. I would definitely recommend a permanent ink so the colours don't run into each other when you spray on your clear finish (Krylon Matte Acrylic spray). I bought the cork feet at Stockade Craft Supplies but you could use rubber, felt or clear plastic ones instead. Or consider covering the whole bottom with self adhesive cork or felt. Quick and easy.