Yellow topaz is the traditional birthstone of November and blue topaz is the traditional birthstone for December. Topaz is also the astral stone for the zodiac sign Sagittarius. The word topaz comes from the Sandskrit word for 'fire'. Ancient Egyptians wore it to protect themselves from injury and the Greeks believed it held the power to increase strength as well as the ability to make its wearer invisible while the Romans believed it had the ability to improve eyesight. Red and pink topaz were commonly found in the jewelry of Russian Czarinas between the 18th and 19th centures. During the Middle ages, topaz was believed to heal both mental and physical disorders as well as preventing death. Topaz has a hardness of 8 out of 10 on the Mohs' Scale of mineral hardness.