After you scrape the mineral specimen with a metal implement, you will see a faint line. But is that line necessarily the result of scratching? The metal instrument might be leaving behind tiny flecks of metal or the mineral might be powdery enough that it leaves behind no trace. To conduct a hardness test properly, you must learn how to identify and eliminate non-scratch markings.
For this exercise, you should use a typical, unpolished sample that you do not mind damaging. Pick an area that isn’t directly on one of the large, flat faces of the crystal and use only very gentle pressure to make one single, quick line across the specimen. Then, gently brush away any loose grains of the mineral, or wipe the test site with your finger or a piece of soft cloth. If the line vanishes after doing this, then it was merely a residue and should not be considered as evidence of a scratch.
Next, you should look at the cleaned test area through a hand lens. Tilt the specimen slightly, so that you can shine a light across its surface. A scratch will still be apparent as a groove or trail that disrupts the normal appearance of the specimen. On the other hand, a powdery residue will sit atop the specimen’s surface, smear when rubbed, or disappear altogether when wiped clean. Also, metal instruments might leave behind gray or silver streaks that look like a pencil line but do not scratch the surface.
To clear up any doubts, try a reverse test. You don’t just ask “Can this metal tool scratch the specimen?” You also ask “Can this piece of the specimen scratch a known material?” Remove any powdery residue from both the specimen and the testing implement, and then try to make a scratch with the specimen against the known material. This will come in handy if you have a rough-grained sample that leaves behind a residue, or if the metal implement you’re testing with is actually softer than you think. Just don’t use the pointed ends of a crystal, because they could break off without producing much evidence either way.
Testing uneven samples is more difficult. Your mineral might consist of many different minerals held together in a matrix, and your metal implement could be dragged over a hard grain, only to fall down onto a softer one when it gets dragged further. Or perhaps your sample has been weathered to the point that it’s already becoming powdery on its own. In any case, you should note where your test was made, and then try to find another spot to re-test, just to be sure. Don’t keep pressing if you get conflicting results, just stop and rethink things.
Don’t write down the hardness as a specific value; instead, write it as a comparison. For example, you might write “Metal tool left residue that wiped away” or “Reference point left permanent groove.” If no tool scratched your sample but another, harder tool did, write down your sample’s hardness as a range between those two values. Always look for luster, streak, cleavage, fracture, and transparency, too, before looking for your sample in a guide book.
When you do a follow-up hardness test, keep your eye on the specimen after the obvious scratch has been cleaned off. The most valuable evidence won’t be the initial mark, but the difference you can see under light from a side angle.
