In the mineral identification questions, we commonly need information in addition to the hardness test suggested in our guidelines. In particular reactivity with acid and streak tests can be important.
It is easy enough to describe how to do a streak test on unglazed ceramic (eg the foot of a plate or cup) but I'm a bit nervous about suggesting people use HCl. The 'muriatic acid' from hardware stores is about 31% according to the web so even that should be diluted 1:2 acid:water. Although I'd be comfortable neutralizing acid with sodium bicarbonate for disposal, I'm not sure I'm comfortable explaining to others - especially since there (worst case) would be varying local regulations.