Carbon occurs in the earth's crust in free state as coal, diamond and ______

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the forms in which carbon occurs in the earth's crust in a free state. We already know two of them: coal and diamond.

2. **Identifying Carbon Allotropes**: Carbon exists in several allotropes, which are different structural forms of the same element. The most common allotropes of carbon include coal, diamond, and graphite.

3. **Listing the Allotropes**:
- Coal: A black or brownish-black sedimentary rock that is primarily made of carbon.
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