Here's why:

* Mohs Hardness Scale: Dolomite has a Mohs hardness of 3.5-4. This means it can scratch a copper penny (hardness 3) but not a steel knife (hardness 5.5).

* Mineral Composition: Dolomite is a carbonate mineral, and carbonate minerals tend to be harder than other types of minerals like gypsum (soft rock).

While dolomite is harder than some rocks, it's still considered relatively soft compared to other hard rocks like granite or quartz.