I saw this on a children's television program and stored it away in the dim and distant recesses of my memory where it was only dredged out recently when Master Chief wanted to make cards.
What you'll need
Eggshells (You'll need way more eggshells than you think)
Normal food dye in whatever colours you want to use (or have)
A bowl for each colour you're using (I used china bowls, I think plastic ones would take on the colouring)
Teaspoons (I used one for each colour but you could use and wash and then use for the next colour)
What to do
Before you want to use the eggshells you'll have to clean them. I rinsed them in water and then put them in a bowl of water to soak, this cleaned them and also made it easier to peel the inner lining off the shell. You need to leave them to dry after this so do this the day before, or in the morning for use in the afternoon.
When they are dry, crush the shells using the back of the teaspoon (you want them small but not crushed into powder).
Put the shells into the bowls and mix in the colouring. Mix it in well, you want the eggshells to take on the colour thoroughly.
Leave the shells to dry, this shouldn't take long depending on how much dye you used (I spread the shells out on some kitchen towel to dry quicker)
The dye will seep through the kitchen towel so don't put them on anything that could stain.
When they're dry you can start making your card!
Depending on how artistic you are you might want to do this freehand or you might want to draw a design on the card to follow.
Glue the area of the card you want to put the eggshells on and then spread a thin layer of the eggshells on. Drizzle some more glue over the top so it sticks really well. If it doesn't look enough then put some more shells and more glue on until you're happy with it.
We did a pirate ship design as Master Chief is a pirate fan (who isn't?) and we added glitter to the blue "sea" for added effect (and because glitter is fab).
We added a little pirate that was cut out of a party cup and a little pirate flag and hey presto! A lovely pirate card.
I was going to tidy it up a bit but Master Chief wanted it just as it was and he's the artist so we left it. The recipient was very impressed!