What treatment can create a golden color for black coral?

Bleaching is a process that can effectively change the color of black coral to a golden shade. During bleaching, chemicals are used to lighten the natural color by removing dark pigments, allowing lighter shades to surface and produce a golden hue. This treatment alters the coral's appearance by stripping it of its darker colors, making it desirable for use in jewelry and decorative items.

Other treatments mentioned, like polishing, typically enhance surface luster and do not alter the inherent color of the material. Calcium treatment generally refers to processes involving the mineral composition, which does not contribute to color changes in coral. Heat treatment can improve clarity or color in certain gemstones but is not typically used for transforming the color of coral to a golden shade. Thus, bleaching stands out as the method specifically aimed at achieving that golden result for black coral.