I found a whole bag of these great multicoloured sea shells at a flea market and for about a year I was debating what to do with them, until I had the idea of using the shells like mosaic tiles. In a nutshell what I did was cover the shells in jointing compound and then partially dig them out again. Here is a more detailed tutorial:
What you need:
a plate or bowl (made of cardboard or wood)
lots of sea shells
joint compound used for mosaics
a paint brush and white paint (which I forgot to include in the picture)
a mixing bowl and some water
Step 1: Figure out how you want to arrange the shells on the plate. If necessary you can break some into smaller pieces with a hammer.
Step 2: Mix the joint compound with water to create a thick paste. Cover the shells (and especially their edges) with that mix.
Step 3: Immediately after that start cleaning off the tops of the shells with water so only the edges are covered with the joint compound. Smooth out the areas between the shells. This might take some time, prepare to feel like an archeologist (at least I did). Let everything dry overnight.
Step 4: Paint the joint compound with white paint. Make sure to blend the paint into the edges of the shells.
I love DIYs, feel free to send me any cool ones you find or made yourself.