This is the number of karats representing 100% pure gold
Note 
that 100% pure gold is very soft and as such can not be used in... [DOC] [TLE] Gold Purity and the Differences Between White and Yellow Gold ... [PAR] We hear a piece of jewelry is pure gold or solid gold or 24 karat gold, but ... 
Karated gold used in engagement rings can be from 100% gold to as low as 37% 
gold.