As described in historical dictionaries

An Etymological Dictionary of Family and Christian Names (1857)

(origin: Local) Garnedd, Welsh, a tumulus; an ancient place of Druid worship. Carnedd, a cairn. Garnet, a precious stone.

Arthur, William (1857) An Etymological Dictionary of Family and Christian Names. New York: Sheldon, Blakeman. Public Domain.


Patronymica Britannica (1860)

GARNET. GARNETT. Said to be a corruption of Gernet.

Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.


BritishSurnames.uk is a Good Stuff website.