It's quite likely that hard disk actually had glass platters. Glass platters still look metallic due to the coating.
However, at least you know that no one could ever try grabbing data from that disk now. :disagree:
I noticed something similar before where I deliberately threw an old HDD on a concrete pavement before popping it in the trash and picking it up to hear it sound like it was filled with pebbles.
It's the poor man's alternative to degaussing a hard disk.