Chalk or clay? Sounds even more ridiculous than sponges -_- A mineral deficiency...? What do you mean
Yeah, some people chew on wood, hair, stones and ice among other stranger things, too. If this has been going on for more than a month I'd go to a doctor as it could indicate you have low iron or zinc levels or you might be pregnant (check this as I haven't read about pica for a while now)
edit: I checked and I was right - still got it lol. But if it's gone on for a month or more you should probably get a blood test to be on the safe side. Go to NHS Choices or patient.info if you want to read more about it. If your blood test comes back normal then enjoy your sponge-chewing. It's weird but it's not harming anyone aha (just make sure they're clean).
Also, do you start licking the screen when SpongeBob appears on TV? U sad, sad creature.
You feel the need to insult people online for absolutely no reason, hiding behind that computer screen. Just to make you feel better about your crappy existence. You sad, sad person :/