Gypsum is an ore of magnesium.

Explanation

### Step-by-Step Solution:

1. **Identify the Chemical Formula of Gypsum**:
- The chemical formula of gypsum is CaSO₄·2H₂O.

2. **Analyze the Components of Gypsum**:
- The components of gypsum include calcium (Ca), sulfate (SO₄), and water (H₂O).

3. **Determine the Main Element in Gypsum**:
- From the chemical formula, it is clear that the primary metal present in gypsum is calcium, not magnesium.

4. **Evaluate the Statement**:
- The statement claims that gypsum is an ore of magnesium. However, since the chemical formula indicates the presence of calcium, this means that gypsum is actually an ore of calcium.

5. **Conclusion**:
- Therefore, the statement "Gypsum is an ore of magnesium" is incorrect. The correct statement is that gypsum is an ore of calcium.

### Final Answer:
Gypsum is not an ore of magnesium; it is an ore of calcium.

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