Steel hardess testing
There are several hardness scales for iron/steel, one of which is the rockwell scale - the higher the number, the harder the material. The material is tested by driving a round tipped punch of a given diameter into the sample with a given force. The depth of penetration shows the hardness.
I got the knowledge of steel hardness testing in Leonard Lee's book The Complete Guide to Sharpening. I highly recommend it, as the best sharpening guide I've read. Leonard Lee is the man who founded Lee Valley Tools and Veritas.
There may be other books on sharpening that are just as useful, but I haven't found them yet. Leonard Lee talks about all kinds of neat stuff in the book. I found it in my local library. I've checked it out several times, and gone in to refer to it even more frequently. My Daddy taught me to sharpen pocket knives when I was in 2nd grade, and Leonard Lee's explanations remind me of the way Daddy taught me, so that may color my judgement. Other folks might find someone else's style more beneficial, which is fine. If we were all the same, it'd be a far less interesting world.