Rosarita is a unique material, which derives its exceptionally rich, red color from the process of gold refining. It is a unique by-product of the 1960s and 1970s gold refining processes. The Alaskan beach sand was smelted for its gold content and the slag by-product was Rosarita, which is essentially a gold-infused glass. The base material for Rosarita is silica (which is quartz particles) mixed with gold at high temperatures.
Old processes prior to 1960 used the old method of open crucible and fire (infused heat) to smelt the sand, leaving behind a glass-like material in the crucible, which is known as slag. This gold slag was used in the 1940s through the 1970s for jewelry, carvings, flint napping, etc. This material known as Rosarita is some of the most exceptional colored gold slag ever produced. Most other slags were mostly red in color but not nearly as vivid as the magnificent Rosarita.
Rosarita is no longer produced because of improved efficiency in gold refinement. Modern processes extract nearly all of the gold out of the sand, leaving an un-remarkable clear glass slag without the deep red color that makes Rosarita so beautiful. [Source]