the.com/karat

a fraction wearing a crown, measuring how pure your gold actually is

means A unit measuring the purity of gold as a fraction of 24, so 24 karat is pure gold and 18 karat is 18 parts gold to 6 parts other metal.

from It traces back to the carob seed — Greek 'keration,' meaning 'little horn,' for the seed's shape — which was once used as a tiny standard weight by goldsmiths and traders. The word traveled through Arabic 'qirat' and Italian 'carato' into European tongues, and English eventually split it into two spellings: 'carat' for gemstone weight and 'karat' for gold purity, the same seed quietly weighing both.

24-part scale: 24 karat means 24 out of 24 parts gold
too soft: Pure 24k bends easily, so jewelers add alloys
carat confusion: Karat is purity; carat is diamond weight
ancient seed: Name traces to carob seeds used as weights
common gold: Most US jewelry is 14k, just 58% gold

for instance

golconda diamonds — indian mines that produced the world's most famous high-carat stones, including the hope diamond at 45.52 carats

koh-i-noor — 105.6 carat diamond, part of british crown jewels, mined in india around 1656

cullinan i — 530.4 carat diamond, largest clear cut diamond ever made, part of british crown jewels since 1907