Last Updated: Nov 22, 2016     Views: 227

From Dr. Robert Brill, Research Scientist Emeritus:

The ruby color can be produced by a number of different colorants: gold, copper (in a highly-reduced chemical state), or cadmium selenide or sulfide. 

Gold ruby glasses date back into the 17th century and even have some (very rare) early predecessors from Roman times.  However, they were also common in the 19th century. 

Copper rubies date back to the Middle Ages.  For example, the ruby glasses in stained glass cathedral window are copper rubies.  That was true for glasses from the 12th century onward and was still true in the 19th century. 

Cadmium selenides and sulfides appeared around the turn of the 20th century and were used for signal glasses in traffic lights, automobile tail lights, and airport runway lamps.

These ruby glasses are so strongly colored that they would appear black in thick sections.  Therefore, they are often used in the form of thin "flashings" over colorless or greenish base glasses.  That way their beautiful ruby colors become visible.