Ruby is the July birthstone – and it’s one of the most coveted of gems. Called the “king of gems” in ancient India, today it can command the highest per-carat price of any colored stone. Here’s what you need to know about this beautiful gem.
Ruby, the July birthstone, is a variety of the mineral corundum that contains trace amounts of the element chromium, which causes its color. The more chromium present, the stronger the color red. Chromium can also cause red fluorescence, which adds to the intensity of the color. Fluorescence is visible light temporarily emitted by some gem materials when they are exposed to high-energy sources such as ultraviolet light (as in sunlight), X-rays or lasers.
Some rubies form in marble, when heat and pressure act on minerals in limestone. These rubies tend to be highly prized, as they have a higher concentration of chromium and little iron, which is an ideal recipe for a bright red color. They also tend to be highly fluorescent.
Other rubies are found in basalt, which forms when molten lava cools and solidifies. Basalt-hosted rubies typically have significantly more iron than marble-hosted rubies and thus are darker. A higher iron content can also mask the fluorescence, eliminating that extra glow of red color.
Still other rubies form in an amphibole-bearing gneiss (a coarse-grained metamorphic rock). These rubies tend to have good color, though they have more iron than the marble-hosted stones and less than those from basalt.