the number refers to the hardness of the rubble, which transfers to gems that you gain from sifting. That's as far as I have gone so far myself sorry, a mate is doing the gemcutting stuff.
I've always wondered if the hardness affected the changes of you getting stuff in general while sifting though. Because some people are able to produce tons of glass it seems; while I'm spending a day digging out rubble boulders, chipping them, then sifting them, and are left with only 1-2 glass to show for it. =_= And I still yet to get a decent mining set so hoping that's my only issue. ^^;