Limonite is not a single mineral, but a mineraloid, which is a naturally occurring solid with no definite chemical composition. Because of this, it can have varying hardness.

Generally, Limonite falls in the range of 4-5.5 on the Mohs Hardness Scale.

This means that it can be scratched by a steel knife (hardness 5.5), but not by a copper coin (hardness 3).