http://tvtropes.org/pmwiki/pmwiki.php/Main/MohsScaleOfRockAndMetalHardness

If you're looking for the scale that ranks the hardness of actual rocks and metals, Wikipedia has you covered. This one is about music.

Basically, a scale for ranking the "hardness" of music, "hardness" being the perceived quality that separates rock from pop, Hard Rock from soft rock, Heavy Metal from Hard Rock, Death Metal from classic metal, noise from Grindcore and industrial metal, and so on and so on.

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As of late, the scale has been expanded to include genres related to rock and metal such as Electronic Music, blues, jazz, and country. The scale also encompasses long-established musical standards such as popnote though the examples provided are, of course, rock-influenced and Classical Musicnote which actually predates rock in being "hard", or "heavy", as proven by many classical pieces made before the rock age. It should be noted that songs from these genres do not usually make it to the scale; those that do are usually similar to or influenced by rock or metal.

Unfortunately, it's kind of a difficult concept to pin down, but trying as hard as possible:

Rough outline of the levels:

Notes:

Suggestion for parental guidance level comparability:

G/U: 13

PG: 45

PG-13/12A: 67

R/15: 89

NC17/18: 1011

Examples: