Below is a brief summary of the information available in the provided SOURCES:
- Subject: The sources confirm the existence of "opal" in the context of product listings (e.g., "opal birthstone necklace").
- Context: The gemstone is marketed as jewelry, specifically necklaces, intended for women.
- Availability: The product is listed on an e-commerce platform (Amazon.com).
- Lack of Detail: There is no information regarding the gemstone's chemical composition, hardness, origin, historical significance, or metaphysical beliefs.
Due to the complete absence of relevant gemological or historical data, a comprehensive article on the topic of opal cannot be generated from these sources.