Origin of the word Carat
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.
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.