A ruby is an extremely strong stone, ranking 9.0 on the Mohs scale of mineral hardness. Natural rubies contain imperfections, including color impurities and inclusions of rutile needles referred to as "silk." These imperfections play an important role in detecting artificial rubies. Heat treatments are very common among most rubies today, conducted prior to the cutting of the stone. However, there are a few high-quality rubies that have not been treated, demanding high prices.