A red ruby is a highly prized, red-colored gemstone, a variety of the mineral corundum, known for its vibrant color and durability, and is valued for its color, clarity, cut, and carat weight. Chromium is the trace element that causes ruby's red, which ranges from an orangy red to a purplish red. The strength of ruby's red depends on how much chromium is present—the more chromium, the stronger the red color. Chromium can also cause fluorescence, which adds to the intensity of the red color.