Ruby is the red variety of corundum, a mineral composed of aluminum oxide (Al₂O₃) and a close relative of sapphire. Its red color comes primarily from traces of chromium, which alter the way the crystal absorbs light, creating hues ranging from pinkish-red to deep red. With a hardness of 9 on the Mohs scale, ruby is one of the hardest gemstones, second only to diamond, making it highly prized in jewelry. Optically, some rubies exhibit fluorescence (often under UV light) that can visually intensify the red, sometimes giving a very vivid brilliance.

Rubies are almost never perfectly "flawless": their inclusions (rutile needles, microfractures, crystals) are veritable natural signatures that also aid in identification. When these fine needles are oriented in a regular pattern, they can produce a spectacular phenomenon: the star ruby, where a luminous star appears on the surface, especially in a cabochon cut. The boundary between ruby and pink sapphire is primarily a matter of shade: the more the stone leans towards pink, the more often it is classified as sapphire, a distinction that also depends on market practices. The term "pigeon blood" is sometimes used to describe a particularly sought-after, vivid, and saturated red, although this remains a commercial description and not a scientific measurement.

Geological origins also influence appearance: some iron-rich rubies appear darker, while others display a brighter red. Treatments are common, especially heating, which can improve transparency and color by altering certain inclusions. As for care, rubies are robust, but it's best to avoid harsh impacts and opt for gentle cleaning with warm, soapy water. Ultimately, a ruby is a unique blend of trace element chemistry, optical phenomena, and geological history—a rare stone where each shade tells the story of a unique formation.