On this date in 1773 a boy was born who grew up to be the mineralogist Friedrich Mohs, eponym of the standard scale of mineral hardness we all learn to this day. What takes some people by surprise about the Mohs scale of mineral hardness is how low-tech it is. A fellow in the Forum found a strange rock and took it into his shop, applying grinding bits with his Dremel tool to check its hardness. That's not what geologists dothey pull out their nail clippers or pocket change and apply elementary scratch tests. More than nine times out of ten, that's all you need. The remaining times, a bit of sandpaper and maybe a piece of fluorite can characterize the hardness of nearly any mineral. The only difficult part might be memorizing the ten standard minerals of the full Mohs scale. This page lists them all, along with links to more information on each one.
Diamond is Mohs hardness 10 Lars Plougmann, flickr.com