Why Are Rubies Red?

A ruby is red because of chromium. Rubies and sapphires are actually the same mineral, corundum, and the only real difference between them is which trace elements got mixed in during formation. When chromium is present in sufficient concentration, the corundum turns red and becomes a ruby. Every other color of corundum, blue, pink, yellow, and the rest, is classified as sapphire.

The Role of Chromium

Pure corundum, with no trace elements at all, is actually colorless. Color comes entirely from tiny amounts of other elements getting incorporated into the crystal structure as it forms deep underground under intense heat and pressure. Chromium specifically absorbs certain wavelengths of light and reflects red back, which is what produces the color the human eye reads as ruby red. The more chromium present, generally speaking, the more saturated and intense the red.

Why Ruby Color Varies

Not all rubies are the same shade of red. Some lean toward a deep, almost purplish red, while others are lighter and more pink-toned. This variation comes from the exact concentration of chromium along with the presence of other trace elements, like iron, which can shift the tone slightly. The most prized rubies tend to have a vivid, pure red with a hint of blue undertone, often described in the trade as "pigeon's blood" red, a color historically associated with rubies from Myanmar, though fine rubies are mined in several other regions as well.

Ruby and Sapphire: Same Mineral, Different Color

Because ruby and sapphire are both corundum, the physical properties that make corundum an excellent gemstone, its hardness and durability, are identical between the two. Corundum ranks 9 on the Mohs hardness scale, second only to diamond, which is part of why both rubies and sapphires hold up so well to decades of daily wear. The color is the only meaningful distinction, and it comes down entirely to which trace element got trapped in the crystal as it formed.

Why This Matters When Choosing a Ruby

Since color is the primary factor separating a fine ruby from an average one, and that color comes from a natural, uncontrolled process happening underground, no two rubies are identical the way two diamonds of the same specifications might be close. Seeing a ruby in person, under consistent lighting, is the most reliable way to judge whether its particular shade of red is the one you actually want.