writing dates on eggs--writing with charcoal hack

I was really bugged about the idea of using a permanent marker or something toxic on a shell of an egg I plan to eat, and so I tried some dumb ideas, fast forward I remembered charcoal and it seems to be working.  It's been a few days and it hasn't worn off, plus I made a cardboard sign last summer and that has sat out on the ground, horizontal, under rain and snow and everything and is still pretty legible.  So I think it's probably going ot last well enough to be clear which eggs are new enough to be worth keeping.   I think this may apply to geese and ducks more than chickens, I'm not sure, I don't have any chickens but if you need to label eggs then here you go.

It's just a piece from a fire I made and had to put out.  Free, non-toxic, edible in fact, and a good color contrast to the eggshell color.  

Maybe this is obvious and everyone already does this but I have never seen it mentioned anywhere.

Hi Joshua,
That is a good idea. I had enough chickens that I could just mark the cartons instead of individual eggs, but I'm going to keep this in mind.

If I'm keeping an egg for some reason (lately because I'm incubating) and need to know when that egg was collected or had something done to it, I use a pencil.

The same basic idea - but the pencil is easy for me, they last for a long time and I use them for all kinds of things. I can see the pencil marks on my colored eggs, especially now that some raptor got my last Cuckoo Maran, so it works.

For other things, the fresh eggs go in a bowl on the counter that I weigh and tally when the bowl gets full. The cartons are marked and rotated in the fridge once they make it there.

I've been known to use food coloring to mark eggs, too. It's useful when I have a bunch of hardboiled eggs so that we eat the oldest ones first - different drops of food color in the boiling water will dye the shell but not the eggs. Usually. There have been exceptions.