Hardness is a measure of a mineral's resistance to scratching. It is determined using the Mohs Hardness Scale, where diamond is the hardest mineral (10) and talc is the softest (1).

* Gabbro: A coarse-grained igneous rock composed primarily of plagioclase feldspar and pyroxene. It has a Mohs hardness of 5-6.

* Limestone: A sedimentary rock composed primarily of calcium carbonate (CaCO3). It has a Mohs hardness of 3-4.

Therefore, gabbro is harder than limestone because it has a higher Mohs hardness rating.