I'm pretty sure no one knows the answer, but I'm gonna try anyway...

In Howard Nemerov's poem "Redeployment" he has this confusing stanza:

The war may be over. I know a man
Who keeps a pleasant souvenir, he keeps
A soldier's dead blue eyeballs that he found
Somewhere- hard as chalk, and blue as slate.
He clicks them in his pocket while he talks.

I don't think eyeballs can harden naturally. Is there some chemical that would cause them to harden? Does he not mean eyeballs? I thought glass eyes, but why would one soldier have two glass eyes? Then I thought marbles, but... I mean, c'mon. Marbles? Could eyeballs have been code for something else?

Help. Any input would be appreciated. This is my poetry thingy, here...