Diamond is the hardest metal in the world. Hence it is widely used in many industrial applications than just in jewelry. Since it is so hard diamond is used to scratch other metals. Of course in the jewelry world a diamond has no match in terms of beauty and value.

The next hardest metals in the world are Corundum and Topaz.

How about other metals?

The following neat infographic from Compound Interest shows the details:

Click to enlarge

Source: The Mohs Hardness Scale: Comparing the hardness of minerals, Compound Interest

Below is a brief excerpt from the infographic description:

Diamond is commonly known to be the hardest material, but how do other minerals compare with each other? That’s the question the Mohs hardness scale, introduced by Friedrich Mohs in 1812, aims to answer. This graphic looks at his scale and where different minerals and other substances appear on it.

Mohs’ scale is a simple way of comparing the hardness of different minerals. The scale is built on comparisons — a mineral that scratches another is designated as having a higher value for hardness. These values are essentially a giant ranking system, in that they’re all relative. There’s no fixed value of hardness between the different numbers in the scale — in fact, diamond at 10 is several times harder than corundum at 9, but corundum is only around twice as hard as topaz at 8.