It's tricky to rank rocks in terms of "hardness" because there are different ways to measure it. Here's a breakdown of some key points and the top contenders:

Hardness Scales and Considerations:

* Mohs Hardness Scale: This is the most common scale, measuring a mineral's resistance to scratching. It uses 10 reference minerals, with diamond being the hardest at 10. However, it doesn't account for other properties like toughness (resistance to breaking).

* Other Hardness Measures: There are other scales that measure different aspects of hardness, like abrasion resistance or indentation hardness.

* Rock vs. Mineral: Rocks are made up of multiple minerals. A rock's hardness is determined by its hardest constituent mineral.

Top Contenders:

Considering the Mohs scale and other factors, here are some of the hardest rocks found on Earth:

1. Diamond: A pure form of carbon, diamond tops the Mohs scale at 10. It is also incredibly tough and resistant to scratching and breakage.

2. Corundum (Sapphire & Ruby): Corundum, with a Mohs hardness of 9, is the second hardest mineral. Sapphires and rubies are varieties of corundum.

3. Topaz: Topaz has a Mohs hardness of 8, making it a very hard and durable material.

4. Quartz: Quartz ranks 7 on the Mohs scale, making it a very durable and common mineral.

5. Feldspar: Feldspar, while not as hard as the previous entries, is still a tough and durable mineral with a Mohs hardness of 6.

Important Note: While diamonds are incredibly hard, they can be brittle and prone to breakage if struck in the right way.

Let me know if you'd like more information about specific rocks or hardness scales!