Geologists do it according to the Mohs Scale of Hardness. There. That's your "hardness" joke for the post. Now down to business, with the hardest (and softest) scale there is.

In 1812, people had just about given up trying to make various precious elements using alchemy, and resigned themselves to scratching what they could out of the ground. Accepting that resources were limited, and couldn't be transmuted into something new meant that each newly-uncovered vein of minerals had to be carefully measured and graded to obtain its true worth. Some people wanted to know if each mineral melted, and some wanted to know which minerals could be molded, but Friedrich Mohs, of Germany, wanted to know what happens when each mineral was scratched. He devised a series of tests which consisted of rubbing one mineral against another and seeing which one came out scratched.

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Once Friedrich had assessed different materials, he picked some common ones as benchmarks for his measuring system. Using ten common elements, he composed a one-to-ten scale which was limited, relative, and slightly arbitrary, but is still used in material classification today.

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Feldspar

7. Quartz

8. Topaz

9. Corundum

10. Diamond

The absolute numbers of the stones don't mean much. Diamonds are four times harder than Corundum and six times harder than Topaz, but no one's adjusting. Instead, people roughly place things like cubic zirconia (8.5) between levels. The only rule that is adhered to is that each mineral can only scratch those minerals below it on the scale.

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For those performing their own Mohs Tests at home, perhaps to test the toughness of an alien spacecraft that landed in their lawn, there are several everyday objects that can give you approximate Mohs values. A fingernail is about a 2.5 on the Mohs Scale, which means you should only blindly reach into a bag of talcum powder or wet plaster of paris. A penny is just under 3, Calcite is usually found in shells and limestone caves. Fluorite gets put in enamels of bowls and other household objects, but it will get cut by a good steel knife, which ranks at about a five and a half. (A tooth, despite all that fluoride, is a good bit tougher than Fluorite and weighs in at a solid 5.) Porcelain, if you take apart your toilet for the cause, is a surprising heavyweight at 7.5, surprising no one who ever chipped a tooth on their toilet. Beyond that, and you get to sandpaper, which is usually coated in corundum. If the spaceship can take more than that, break out the diamond jewelry. Any tougher than that and you just have to hope they're friendly. And a fan of Mohs testing being done on their shiny new spaceship.

[Sources: The Mohs Scale: From Talc to Diamonds, Mohs Hardness Test.]