The word carat comes from the Carob Mediterranean tree whose seed was used for centuries as the standard of weighing precious gems. Each carob seed weighs approximately one-fifth of a gram. A carat is one-fifth of a gram.
Jewelry & Diamond Industry Dictionary of Terminology
Zircon is a colorful gem with high refraction and fire that is unfairly confused with cubic zirconia.
Colorless zircon is known for its brilliance and flashes of multicolored light, called fire. These zircon properties are close enough to the properties of diamond to account for centuries of confusion between the two gems.
Zircon occurs in an array of colors. Its varied palette of yellow, green, red, reddish brown, and blue hues makes it a favorite among collectors as well as informed consumers.
Color: Blue, red, yellow, orange, brown, green
High: 1.925 to 1.984 (+/- 0.040)
Medium: 1.875 to 1.905 (+/- 0.030)
Low: 1.810 to 1.815 (+/-0.030)
Birefringence: 0.000 to 0.059 (low to high)
Specific gravity: 3.90 to 4.73
Mohs Hardness: 6 to 7.5 (low to high)
Zircon is a birthstone for the month of December, along with turquoise and tanzanite.
Sources for these definitions are GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.