The use of diamond as a gem is due to its

Options

A. extreme hardness
B. poor conductance
C. low density
D. high-refractive index

Explanation

**Step-by-Step Solution:**

1. **Understanding Diamond's Properties**:
- Diamonds are known for several key properties that make them unique. These include extreme hardness, poor electrical conductivity, low density, and a high refractive index.

2. **Extreme Hardness**:
- Diamonds are the hardest known natural material. This hardness makes them durable and resistant to scratching, which is a desirable quality for gemstones.

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