How would a collector of minerals determine the hardness of a unknown mineral specimen?
It's called the Mohs test. You take specimens of known hardness and try to scratch the unknown specimen with a pointed edge of the known specimen. If you can scratch the unknown mineral it is softer, if not it's harder. If you have several mineral samples of known hardness you can narrow down the unknown quickly.
A minerals hardness is its relative ability to scratch or be scratched by other minerals. Diamond, for instance, can scratch all other minerals because of its hardness. Streak is the color of the mineral when powdered. This is usually accomplished by the streak test (swiping the mineral across an unglazed porcelain surface) which reveals a mineral's streak color, which may differ from the color of the specimen being tested.