fine black grit on eggs

[ddelancy]

The last three eggs that one of my hens has laid have been covered with a fine sandy black grit. The eggs are fine otherwise, and the grit wipes right off. The hens seem fine too. I am not sure which one is laying the gritty eggs. All the hens are about 9 months old. Has anyone seen this before?

 

[smudge]

I've not had black grit, but I've had grayish deposits. Doesn't sound like it's calcium deposits if it's black. And I'll assume there's nothing black in the nesting box (dust) transferring onto the egg while wet...