Fineness

To increase the strength of precious metals such  as gold and silver, they are often alloyed with  other  metals.  The  percentage  of  the  precious  metal in the final alloy is known as its level of  fineness. 

In  most  countries  it  is  common  to  express fineness in parts per thousand. For example 9k gold is 375 parts per thousand (37.5% gold) and is therefore hallmarked 375. The reason why it is  then referred to as 9K gold and not 375K is that  pure gold in the karats measurement system is  24K and therefore 37.5% of 24k is equal to 9K.