Ruby is composed of corundum, the same material which sapphire is composed of. The name 'corundum' originates from the ancient Sanskrit word 'kuruvindam'. Ruby is the red gem-quality variety of corundum, while all other colors are referred to as 'sapphire'; both gem types are ranked 9 on the Mohs scale of mineral hardness, making them the second hardest natural known substances on earth. Red ruby, along with sapphire, emerald and diamond, is grouped as one of the 'precious four' gems, although the use of the terms precious and semi-precious gems is no longer recognized by most professional gem and jewelry organizations, including the AGTA (American Gem Trade Association). Red ruby gems can be found in various shapes and cutting styles, including rounds, ovals, pears, trillions and fancy hearts. Oval-shaped rubies are most common because oval shapes are typically cut to maximize carat weight, and since most gems are bought and sold on a price per carat basis, the shape greatly affects the end price for retail consumers. Fine quality red rubies can be very expensive, especially gemstones with top color, clarity and cut. Heavily treated rubies, which usually are sourced from Africa, are very affordable gems, especially due to the introduction of fracture-filling treatment, many rubies can be found in very large gem sizes, even gems weighing 10 carats or more.

Most experts agree that ruby from Myanmar (Burma) is the finest ruby available, particularly "pigeon's-blood" ruby mined from the Mogok region of Burma. Red rubies mined from the Mong Hsu region of Burma are also very popular and they often exhibit a slightly bluish or purplish hue. Mong Hsu rubies weren't as well-received by international markets when they were first introduced to the ruby markets just before the turn of the century, especially when compared to the top color seen in the Burmese ruby gems sourced from Mogok. However, after processing them with routine heat treatment, Thai gem suppliers discovered that Mong Hsu Burmese rubies can exhibit excellent color and clarity, often comparable to that of the Mogok Burma ruby. Red star rubies are also a rarity and very valuable, particularly star ruby with sharp, clear asterism. Many of the star rubies available today are sourced from Thailand, as well as Sri Lanka (Ceylon) and Tanzania.