______ is a crystalline form of allotrope of carbon which is a bad conductor of heat and electricity.

Explanation

**Step-by-Step Solution:**

1. **Identify the Allotropes of Carbon**: Carbon has several allotropes, including diamond, graphite, fullerenes, and graphene. We need to focus on the crystalline forms.

2. **Understand the Properties of Diamond**: Among the allotropes of carbon, diamond is known for its crystalline structure. It forms a giant three-dimensional network.

3. **Valency of Carbon**: Carbon is tetravalent, meaning it has a valency of 4. This means each carbon atom can form four bonds with other carbon atoms.

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