Can a material generally score or cut itself by hand? I'm wondering if a given solid material can, in general, score or cut the same material, when applied by (at most) human muscular strength.
I've tried searching for this online, but it seems like a difficult-to-express search target. For example, the site Answers.com has a brief entry:

Q: What mineral can scratch diamonds? A: The mineral diamond can scratch diamonds.

There's no further elaboration or justification at that site.
My question differs from prior somewhat-related questions on SE Physics, because those answers all end up addressing cases where the cutting material is accelerated to very high (possibly relativistic) speed. The present question is only about whether a limited amount of solid material, hand-held by a person, and applying only normal muscular strength, can generally visibly score or cut the same kind of material, within a few seconds to a minute of time (but assume the cutting material might be sharpened to a high degree).

*

*Is it possible to cut harder material with a less hard material?

*Can a tomato pierce a hole in a steel plate if only the tomato is travelling fast enough?
Is there any general rule for what kind of solid materials can cut themselves in this way?
A: The material quality you're looking for is hardness. The Mohs Hardness Scale is an empirical table of minerals sorted by hardness. Two materials are said to be the same harness if each can scratch the other. A homogeneous or mostly homogeneous solid, like diamond, has an identifiable hardness. A nonhomogeneous solid, like concrete, does not.
A solid-liquid mixture, like a tomato, or a solid-gas mixture, like aerogel, does not. Nonhomogeneous Solid mixtures like concrete will scratch like their hardest component and be scratched like their least hard component. Solid-liquid and solid-gas mixtures may also crush, tear, or burst, unrelated to scratching.
So: if an object has a well-defined hardness, it can scratch itself because it has the same hardness as itself. And an object has a well-defined hardness if it is a homogenous solid.
