Hardness is one of the more distinctive properties that minerals possess and relatively easy to determine. Every mineral is assigned a hardness number between 1.0 and 10.0. This scale was designed by the German mineralogist Frederich Mohs (1773-1839) in 1812 and is therefore known as Mohs Scale. What Mohs did was to assign a common representative mineral to each whole number with 1 being the softest known mineral and 10 being the hardest.
1 - Talc (softest)
2 - Gypsum
3 - Calcite
4 - Fluorite
5 - Apatite
6 - Orthoclase
7 - Quartz
8 - Topaz
9 - Corundum
10 - Diamond (hardest)
Generations of geology students have been tortured by evil professors who make them memorize Mohs Scale minerals and there are a number of mnemonics that have been devised over the years. The one told to us by my mineralogy professor many years ago was "Texas Girls Can Fuck And Other Queer Things Can Do." In these more politically correct days, I tell my students "The Girls/Guys Can Flirt And Other Queer Things Can Do." I've even seen "The Geologist Can Find An Ordinary Quartz Tourists Call Diamond" and "Two Giddy Chickens Found A Friendly Quiet Turtle Cooking Dinner." Hey, whatever works - sometimes the most obscene mnemonics are the ones you'll remember 30 years later!
While you can puchase hardness testing tools, the easiest way is to simply amass a collection of 9 small mineral samples representing the hardnesses above (our geology lab testing kits do not have diamonds in them for obvious reasons!).
Usually these kits also include a piece of glass (H=5.5 on Mohs Scale) and a piece of copper (H=3.5). Other objects you can use to test hardness are a penny (H≈3.5), a steel file (H≈6.5), or your fingernail (H=2.5) - female students with manicured nails are sometimes reluctant to go around scratching minerals however.
So, the procedure I usually give my students is to first try to scratch the glass plate with the mineral. If it scratches the glass it's harder than 5.5 or so and if it doesn't it's less. If it's less, I would then test it with my fingernail. Then the copper strip or penny. Then I would go to the mineral hardness kit to narrow it down. For many minerals in the introductory geology lab, testing with a fingernail and glass plate is usually all you need to narrow it down.
Hardness testing is often useful in the field as well. Veins (mineral-filled fractures) like those below are commonly either calcite (CaCO3) or quartz (SiO2) - two very different minerals that can look very similar at times (especially for students).
A quick test with a knife blade, however, will easily reveal which mineral it is (geologists are typically always armed with a knife). Calcite has a hardness of 3, quartz has a hardness of 7, and a knife blade is typically around 5.5-6. If it scratches easily with the knife, it's calcite. There are a number of common minerals that may look similar but are easily distinguished by their hardness.
One interesting thing about Mohs Scale is that it's not linear. In other words, the difference in hardness between 4 and 5, for example, is not the same as the difference in hardness between 8 and 9. Diamond, at 10, is actually off the chart!
This really doesn't matter too much since we're not testing many diamonds in my lab!
What are we actually testing when we look at a mineral's hardness? Hardness is actually related to a number of things in minerals. The size of the atoms comprising the mineral, their bond lengths, and the type of bonds (e.g. covalent, ionic, van der Waals) all plays a role. What we're really testing is resistance to abrasion since we're scratching the minerals to determine their hardness. Here's a good short article on the topic.