The Website TVTropes has published an article here that recommends a hardness scale for science fiction. For some reason, a particular author isn’t credited, so I guess “staff” is responsible for this wonderful suggestion. For anyone not up on their geology, Mohs Scale of Mineral Hardness ranks the scratch resistance of different minerals by using harder minerals to scratch softer ones. This scale was developed in 1812 by German geologist Friedrich Mohs. Regardless of lack of precision, the test is simple and very effective for geologists with only a field kit who are trying to identify piles of dirty rocks.

So, on to the Mohs scale for SF: Because there is often some contention about whether SF is really hard or not, the staff at TVTropes proposes to scratch SF stories with something like a piece of quartz to see what rubs off. Here’s the scale they’ve come up with:

1.0 Science in Genre Only: The work is unambiguously set in the literary genre of Science Fiction, but is not scientific. Examples: DC and Marvel universes, Douglas Adams’ The Hitchhiker’s Guide to the Galaxy.

2.0 World of Phlebotinum: The universe is full of Applied Phlebotinum, but it’s dealt with in a fairly consistent manner. Examples: E. E. “Doc” Smith’s Lensman series, Star Trek: The Original Series.

2.5 Subclass of WOP: Stories are generally sound, but the physics aren’t our own. Often a philosophical exploration of a concept no longer considered true, or never true in the first place. Tricky to classify. Examples: Aristotelian physics, two spatial dimensions.

3.0 Physics Plus: Multiple forms of Applied Phlebotinum, but the author tries to justify these with real and invented natural laws. Examples: David Brin’s Uplift series, Battlestar Galactica (2003).

4.0 One Big Lie: Provides counterfactual physical laws and then explores the implications of these principles. Examples: Alan Dean Foster’s Humanx Commonwealth, Robert Heinlein’s Farnham’s Freehold.

4.5 One Small Fib: Stories have a single counterfactual device (e.g. FTL travel), but the device is not a major plot element. Examples: Hal Clement’s Mission of Gravity, Close to Critical.

5.0 Speculative Science: Science is genuine speculative science or engineering, and the author’s goal is to make as few errors with respect to known fact as possible. Examples: Robert L. Forward’s Rocheworld, Robert Heinlein’s The Moon Is a Harsh Mistress.

5.5 Futurology: Stories that try to predict the future, extrapolating from current technology. Examples E. M. Forster’s The Machine Stops, Jules Verne’s From the Earth to the Moon.

6.0 Real Life (aka Fiction in Genre Only): Also known as non-fiction. Examples: The Apollo Program, World War II, Woodstock.