Ruby / Corundum:
Ruby / Corundum is a crystalline form of aluminum oxide (Al2O3) typically containing traces of iron, titanium, vanadium and chromium. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen. Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral.
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.