Well Here are the Images of this mirror blank.
I could not get the super fine pattern that is on the surface of the glass
no matter what I tried. It appears to have been diamond generated.
There are very light swirls that go from center hole to edge.

The Mirror is very fine ground Grit equivalent?  Not too sure. But
as seen here you can almost see through it.

The Physical dimensions are

8" diameter with a 2.6" center hole, and is 1.375" thick
The focal length is a F-4 (24")

 A slightly different angle

Then just a slight bit of spit and it gets nearly transparent!

So, If this was yours,  what would you do with it?
Visit The Original Rolling Ball Clock.
Idle-Tyme
http://www.idle-tyme.com