Ruby, the striking red variety of the mineral corundum, is one of Earth's most treasured and durable gemstones. Known for its vivid red hue, ruby shares its origins with sapphire, another remarkable member of the corundum family. Pure corundum is naturally colorless, but the presence of trace elements such as chromium, iron, titanium, or vanadium infuses these minerals with their rich colors. In the case of ruby, it is the element chromium that bestows the gem with its deep red color, making it one of the most coveted gemstones in the world.

On the Mohs scale, which measures the hardness of minerals, ruby ranks just below diamond with a score of 9. This exceptional hardness makes ruby not only a symbol of beauty but also a gemstone that is well-suited for daily wear. Interestingly, only the red variety of corundum is entitled to be called a ruby, while all other colors are classified as sapphires. This close relationship between ruby and sapphire was only fully understood in the early 19th century. Before this time, red garnets and spinels were often mistaken for rubies, leading to notable misidentifications in history, such as the "Black Ruby" and the "Timur Ruby" in the British Crown Jewels, both of which are actually spinels.

Ruby is primarily composed of aluminium oxide with chromium and other trace elements, and its formation is nothing short of a geological marvel. However, finding rubies of fine colour and clarity is a rare occurrence. Paradoxically, the very element that gives ruby its captivating colour—chromium—also contributes to the gem's scarcity. During the gem’s formation deep within the Earth, chromium often caused fissures and cracks within the crystals, limiting the growth of perfect ruby specimens. As a result, rubies larger than 3 carats are exceedingly rare, and those with minimal inclusions can command even higher prices than diamonds of the same size at auction.

Some rubies are blessed with a silky sheen, known as the "silk" of the ruby, a result of fine rutile needle inclusions within the stone. Occasionally, an even rarer phenomenon occurs—star rubies. These enchanting gems display a star-shaped pattern caused by rutile needles arranged in such a way that they create a light effect known as asterism. When star rubies are cut into a half-dome shape, or cabochon, a six-rayed star appears to glide across the stone’s surface as it moves. The value of star rubies depends on the beauty and intensity of their colour, the sharpness of the star, and its centred position within the gem. A fine star ruby is a true rarity, with fully formed rays that extend to the edges of the stone, adding to its allure.