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
A horologist is someone skilled in the practice or theory of horology. Traditionally, a horologist is a maker of clocks or watches.
Sources for these definitions are GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.