For what purpose are the black diamonds used ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Black Diamonds:**
- Black diamonds, also known as carbonado, are a type of diamond that is characterized by their dark color. Unlike traditional diamonds, which are typically clear or white, black diamonds have a unique appearance due to the presence of numerous inclusions and a different crystal structure.

2. **Cost Comparison:**
- Black diamonds are generally less expensive than white diamonds. This price difference is primarily due to lower consumer demand for black diamonds compared to the more traditional white diamonds.

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