The hardest rock is
💎 Understanding Hardness: Rock vs. Mineral
This question is slightly tricky because it mixes rocks and minerals. It's crucial to understand the difference and how hardness is measured.
Mineral: A naturally occurring, inorganic solid with a definite chemical composition and a specific crystalline structure. (e.g., Quartz, Diamond, Talc).
Rock: An aggregate or mixture of one or more minerals. (e.g., Granite is a rock made of quartz, feldspar, and mica minerals).
Mohs Scale of Hardness: A scale from 1 to 10 that ranks minerals based on their ability to scratch one another. A mineral can only scratch other minerals that are softer than it. 1 is the softest, and 10 is the hardest.
🔬 Detailed Analysis of the Options
B. Diamond
This is the correct answer. Diamond is a mineral composed of pure carbon. It is the hardest known natural substance on Earth, ranking 10 out of 10 on the Mohs scale. While the question asks for the hardest "rock," diamond is the hardest material listed by a very large margin.
D. Quartz
Quartz is a very hard mineral, ranking 7 on the Mohs scale. It is a major component of many hard rocks like granite and quartzite. If diamond were not an option, a rock made of quartz (like quartzite) would be the hardest rock you could infer from the list.
A. Marble
Marble is a metamorphic rock composed primarily of the mineral calcite. Calcite has a hardness of only 3 on the Mohs scale, meaning marble is a relatively soft stone that can be easily scratched by a steel knife.
C. Talc
Talc is a mineral that is famous for being the softest mineral on the Mohs scale, with a hardness of 1. It is so soft you can scratch it with your fingernail.
📊 The Mohs Scale of Hardness
| Hardness | Mineral | Common Test |
|---|---|---|
| 10 | Diamond | Scratches everything |
| 9 | Corundum | Scratches quartz |
| 8 | Topaz | Scratches quartz |
| 7 | Quartz | Scratches steel and glass |
| 6 | Orthoclase (Feldspar) | Scratched by steel file |
| 5 | Apatite | Scratched by a knife |
| 4 | Fluorite | Scratched by a knife |
| 3 | Calcite (in Marble) | Scratched by a copper coin |
| 2 | Gypsum | Scratched by a fingernail |
| 1 | Talc | Softest, feels soapy |
💡 Study Tips
- Mineral vs. Rock: Be careful! The question asks for a "rock" but lists mostly minerals. Diamond is the hardest *substance* listed.
- Top and Bottom of the Scale: Remember the two extremes of the Mohs scale: Talc is #1 (softest) and Diamond is #10 (hardest). This helps you eliminate two options immediately.
- Quartz is the "Hard Rock" Standard: Remember that Quartz is a 7. It's the key mineral that makes rocks like granite and quartzite very hard and durable for construction.