You could probably get an inexpensive kit from any place that supplies materials for 8th grade Earth Science classes. Try Edmund's Scientific, for starters.
It won't be as accurate as a lab test, but it will get you in the balpark.
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928
"I'm searching for the questions, so my answers will make sense." - Stephen Brust
What's the mineralogy of your coarse aggregate? If it's quartz then the Mohr's hardness is 7. If it's limestone (calcite) use 3. But if it's dolomite you'd be better to use 3.5 or 4. It talkes a weak solution of HCl to determine the difference between calcite (bubbles readily) and dolomite (bubbles only in knife scratch). If you have feldspar (plagioclase, orthoclase, microcline, labradorite - wouldn' that be cool looking) use 6. Bear in mind that a pocket knife will scratch feldspar but not quartz.
Use something you know as a gauge...for instance, if you can scratch the aggregate with a piece of ordinary glass, its Mohs hardness is likely less than 5. If you can scratch it with a common steel knife blade, the Mohs hardness is less than 7. If you can scratch it with your fingernail, the Mohs hardness is less than 3. If you can scratch it with your girlfriend's new diamond, the Mohs hardness is less than 10.
Somewhere in there you should be able to get close.
