...................... is a colourless, crystalline and purest form of carbon.

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Question**: The question asks for a specific form of carbon that is described as colorless, crystalline, and the purest form of carbon.

2. **Identifying Forms of Carbon**: Carbon exists in several allotropes, which are different structural forms of the same element. Some common allotropes of carbon include graphite, charcoal, and diamond.

3. **Analyzing the Allotropes**:
- **Graphite**: This form of carbon is opaque and has a layered structure, making it black and not crystalline in the colorless sense.
- **Charcoal**: This is an amorphous form of carbon and is not crystalline or colorless.
- **Diamond**: This is a crystalline form of carbon that is known for its clarity and brilliance. It is colorless and is considered one of the purest forms of carbon.

4. **Conclusion**: Based on the analysis, the answer to the question is "diamond."

**Final Answer**: Diamond is a colorless, crystalline, and purest form of carbon.

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