Granite and diorite are two types of igneous rocks that are often confused with one another. They have similar appearances and compositions but there are some key differences between them. One of the most common questions is whether diorite is harder than granite.
On the Mohs scale, granite has a hardness of 6 to 7. This means it is relatively hard and difficult to scratch. Granite gets this hardness from its quartz and feldspar minerals which both rate around 6 to 7 on Mohs scale.
Diorite has a hardness of 5 to 6 on the Mohs scale. This makes it slightly softer than true granite. The main minerals in diorite are plagioclase feldspar and pyroxene or hornblende. Plagioclase rates around 6 on Mohs scale while pyroxene and hornblende are typically 5 to 6. The combination of these minerals gives diorite a hardness in the 5 to 6 range.
While the hardness ranges of granite and diorite overlap, granite is generally harder than diorite. There are a few reasons for this small but noticeable difference:
- Granite contains more quartz than diorite. Quartz is one of the hardest common minerals at 7 on Mohs scale. The higher quartz content of granite makes it tougher.
- Diorite contains more plagioclase feldspar than granite. Plagioclase is somewhat softer than potassium feldspar, which is abundant in granite. This lowers diorite's overall hardness.
- Pyroxene and hornblende found in diorite are softer than the feldspars that dominate both rocks. As mafic minerals, they contribute to a lower hardness in diorite.
So in most cases, granite rates slightly higher on the Mohs scale, meaning it is more resistant to abrasion and scratching than diorite. But the differences are subtle. When observing the two rocks, the scratch test may not reveal a clear hardness distinction between them.
The hardness similarity between granite and diorite comes from their shared composition. Both contain abundant feldspar minerals combined with quartz or darker mafic minerals. The ratio of those minerals is what ultimately determines the fine-scale hardness differences between the two igneous rocks.
In practical use, granite and diorite have similar durability. Both can be polished to a shine and used in flooring, countertops, architecture, and sculpture. Granite is more frequently chosen for these uses due to its prestige, lighter colors, and patterned appearance. But in terms of hardness alone, diorite can make an effective substitute for projects that require a durable igneous rock.
So while granite narrowly edges out diorite in hardness, the differences are slight enough that both can serve comparable roles as hard, durable building and landscaping stones.
