I'm not sure if these pieces are actually geodes, druzies, or agates. I find them on eBay under searches using all of those keywords so it's never really been clear. I know that geodes have little mineral formations/crystals etc. and some of the pieces I'm using may or may not fall under the agate name used, but because I can't find a definite answer, I'm going with "agate".
These are great easy to make magnets. They make wonderful gifts and are very pretty.
Step 1: BoM
Agate slices (I get mine from eBay)
Magnets (craft store)
Metallic paint & brushes (craft store)
Step 2: Paint
Carefully paint the edges of the slice and allow to dry. Depending on the piece, you may want to paint the back as well but this is optional.
Step 3: Glue
When your slice is dry, glue the magnet to the back and allow to dry/cure fully before use. Some of my magnets are self-adhesive and I still put epoxy on them.