On Tue, 08 May 2012 09:17:52 -0700, Oren wrote:

I'm really interested in what this "coating" is.

I'm pretty sure it's calcium-something.
Today I will drop a chunk into 29% muriatic acid (with glasses).
I'll see if it bubbles.

I'm not sure 'what' that tells me if it does - but someone suggested it
somewhere so that's what I'll do and report back.