Rubies are one of the few gemstones that can give diamonds a run for their money. Just consider the Van Cleef & Arpels “scarf” necklace the Duke of Windsor presented to the Duchess on her 40th birthday, in 1936 — set with diamonds and dripping with rubies, a testament to the deep-red gemstone’s power — or the slippers encrusted with 4,600 rubies that Harry Winston made to commemorate The Wizard of Oz’s golden anniversary. July babies have permission to adorn themselves with this beautiful red stone even when it’s not their birthday.

Rubies are considered precious stones — along with diamonds, sapphires and emeralds — and have a hardness of 9.0 on the Mohs Scale, surpassed only by Moissanite and diamonds. They are composed of corundum, a colorless mineral that is also the basic material of sapphires. In the July birthstone, the red of the gemstone — and the various hues seen in sapphires — are produced by the presence of trace elements. In the case of a ruby, this element is chromium. Rubies range in color from vermilion to a violet red. They are also pleochroic, which means that a stone’s hue can vary depending on the direction of viewing. The most sought-after color is pigeon’s blood: pure red with a hint of blue.

Rubies that are hosted in dolomite marble are the most prized: Because the marble is low in iron, so are the rubies, resulting in a more intense color. Rubies found in basalt, which has a higher iron content, are generally darker and less intense.

When shopping for antique and vintage ruby jewelry, remember that the 4Cs of selecting the perfect diamond — color, clarity, cut and carat — also apply to rubies.

According to the Gemological Institute of America, the color of the July birthstone should be a vibrant to slightly purplish red, and the stone should be clear and inclusion-free. Accordingly, the cut should show off its color and clarity. As for the last criterion, fine rubies more than a carat in weight are rare. Often, large rubies are more expensive than diamonds of comparable weights.

Find a collection of ruby necklaces, ruby rings and other accessories on 1stDibs.