ATM Another Tile question

[NGC704@aol.com]

Greetings..!
For my first mirror project, an 8" kit from W-B, I have made a cement tool
which I've coated with epoxy , and to which I have attached unglazed white
(what I thought to be) ceramic tiles with same..
I planned on trimming the edges of the tiles down and then beveling them using
a belt sander, but when applied, gave off sparks as if it were iron, and got
very little results in sanding (grinding?) action.. Got the same results with
the bench grinder.. Both tests were  somwhat brief as these tiles look way too
hard to be the right kind..!
Is it possible I got the wrong kind of tiles??  If not, I'd be at a loss as to
how to trim the rascals..
TIA,
Kurt