Sapphire is a variety of corundum, an aluminum oxide (Al₂O₃) renowned for its hardness of 9 on the Mohs scale, second only to diamond. Sapphire is often associated with blue, but in reality, the term “sapphire” refers to almost all the colors of corundum… except red, which is called ruby. The colors come from trace elements: iron and titanium generally produce blue, while chromium can produce pinks and certain purples, and combinations of elements lead to yellows, greens, or oranges. This coloration by “trace elements” is an excellent example of chemistry: a tiny amount of atoms is enough to transform the appearance of a sapphire.

Sapphires can exhibit pleochroism, meaning their color varies depending on the viewing angle, a characteristic lapidaries take into account when cutting. Some stones also display color zoning (lighter/darker bands or zones), a veritable record of their growth. When a sapphire contains fine, aligned rutile needles, it can become a star sapphire: a luminous star appears on the surface, especially in cabochon cuts. Among the most sought-after varieties are the "royal" blue sapphire and the padparadscha, a rare shade between pink and orange, highly prized by collectors.
Like many gemstones, sapphire is often heated in a controlled manner to enhance its color and transparency, a common treatment in the industry. Its geological origin also influences its appearance: depending on the deposit, saturation, clarity, and inclusions can vary significantly. Inclusions (crystals, veils, "silks") are not merely "flaws": they help gemologists distinguish a natural sapphire from a synthetic one. While it is a very durable stone, cleaning with warm soapy water and a soft cloth remains the safest approach. Ultimately, the sapphire is a gemstone where the physics of light, the chemistry of the elements, and geological history converge, resulting in an almost infinite range of varieties.