Only _____ can cut another diamond.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks who or what can cut another diamond. This implies we need to consider the properties of diamonds and their hardness.

2. **Properties of Diamond:**
Diamonds are known to be the hardest natural substance on Earth. They are an allotrope of carbon, which means they are a different structural form of the same element.

3. **Cutting Mechanism:**
To cut a material, the cutting tool must be harder than the material being cut. Since diamonds are the hardest known material, they cannot be cut by any substance that is softer than themselves.

4. **Conclusion:**
Therefore, the only material that can cut another diamond is another diamond. This is because only another diamond possesses the hardness required to cut through a diamond.

5. **Final Answer:**
Only **diamond** can cut another diamond.

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