Corundum is an ore of aluminium.

Explanation

**Step-by-Step Solution:**

1. **Identify Corundum**: Corundum is a mineral composed primarily of aluminum oxide (Al₂O₃). It is important to recognize its composition to understand its significance as an ore.

2. **Chemical Formula**: The chemical formula of corundum is Al₂O₃. This indicates that it contains aluminum (Al) and oxygen (O) in a specific ratio, which is essential for identifying it as an aluminum ore.

3. **Properties of Corundum**: Corundum is known for its hardness, which is rated at 9 on the Mohs scale (not 10 as mentioned in the video). This makes it one of the hardest natural substances, second only to diamond.

4. **Crystal Structure**: Corundum typically forms in a crystalline structure, which contributes to its hardness and clarity. This crystal clear appearance can also be a distinguishing feature of corundum.

5. **Conclusion**: Since corundum is primarily composed of aluminum oxide, it is classified as an ore of aluminum. Therefore, the statement "Corundum is an ore of aluminum" is true.

---