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
Decorative material, usually a mineral, prized for some or all of the qualities of beauty, durability, and rarity. It is used synonymously with "gem," and "stone."
Sources for these definitions are Cally Hall, GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.