Geology.com provides a copy of the Mohs hardness scale, a standard classification chart for the hardness of metal. The chart runs from talc, labeled the softest mineral with a Mohs rating of 1, to diamond, the hardest metal rated at 10 on the Mohs scale, as Geology.com explains
Scientists test the hardness of minerals by using one mineral to scratch another and observing the results. Most common minerals are rated according to the Mohs scale, with some showing a range on the scale rather than a definitive numerical rating. For instance, while quartz has a rating of 7 on the scale, the softer mineral gold's rating is placed within the range of 2.5 to 3, according to Geology.com.