Spinel is the magnesium aluminum member of the larger spinel group of minerals. Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, blue, green, yellow, brown, or black.
Rhodium is a noble metal with a whitish gray appearance, and it is typically applied to metal through electroplating. A plating of rhodium is applied over either sterling silver or alloys to give a piece of jewelry a bright, glossy and long-lasting finish.