Ruby is the most valuable variety of the corundum mineral species, which also includes sapphire.

Ruby is a prestigious variety of the corundum mineral species, which also includes sapphire. Esteemed for its vibrant colour and rarity, ruby commands some of the highest prices per carat in the coloured gemstone market.

In its pure form, corundum is colourless. The rich red hue of ruby is due to trace amounts of chromium integrated into its crystal structure. This chromium can impart a range of red tones, from orangy-red to purplish-red. The intensity of the red colour depends on the concentration of chromium—more chromium results in a deeper red. Additionally, chromium can cause fluorescence, enhancing the gem's color and overall brilliance.

Notable rubies, such as those from Myanmar, the Himalayas, and northern Vietnam, typically form in marble. These rubies are found in irregular layers within the marble, which itself forms from the metamorphic transformation of limestone under heat and pressure during mountain-building processes. The low iron content in marble means that rubies found in these deposits are often remarkably intense in colour.

Marble-hosted rubies are also known for their strong fluorescence under ultraviolet light, including sunlight, which can further enhance their vivid red appearance and boost their value.

Conversely, rubies from basalt deposits often contain higher levels of iron, which can result in darker, less vibrant colours. The increased iron content can also mask the red fluorescence, reducing the extra glow seen in marble-hosted rubies.