This is a really neat technique that uses permanent markers, like Sharpies, and rubbing alcohol to re-flow the inks and make all sorts of mesmerizing swirly effects on the tile. Tiles are a popular material for creating DIY coasters. Be sure to seal the ink with a spray sealer as a final step to protect your work.
The best part about this is if you don’t like the design you end up with, you can always add more alcohol drops to re-flow your ink. Or simply wipe your tile clean with some acetone. Note that acetone is a pretty serious chemical, and should only be handled by adults, and wearing gloves is highly recommended.
Try adding a vinyl cut out as a resist layer before you add the ink. Or use an embossing stamp pad and stamp like this snowflake coaster from She Zig Zags.
Get the full how-to for this project over on the Sharpie Blog.