"Spider web black fossil coral" is agatized fossil coral (a type of gemstone, not true coral) that exhibits a black color with white or gray intricate patterns resembling a spider web. It is valued for its unique aesthetic and perceived metaphysical benefits. 
Origin and Formation
Spider web black fossil coral is not a type of living coral, but rather an ancient fossil where the original coral skeleton has been replaced over millions of years by minerals, primarily agate (a form of chalcedony). This process, called agatization, preserves the intricate skeletal structure of the coral as a durable, stone-like material. 
Jeweler Uses
Jewelers prize spider web black fossil coral for its unique, natural patterns that add depth and a sense of earth history to a piece. 
Believed Benefits (Metaphysical)
In holistic and spiritual practices, black fossil coral is primarily associated with grounding and protection. These claims are not scientifically proven and should not replace medical advice.