Well the 18K would indicate that it is 18 carat gold, I am not sure of the J50 but it sounds like a manufacturers part number for that type of ring.
The traditional way to describe the proportion of gold in alloys is in "carats". A carat is 1/24, that is 1 part in 24 by weight.
18 carat is therefore 18/24 or 3/4 gold. In recent years it has become common to refer to alloys millesimally, that is in "parts per thousand". Eighteen carat gold is therefore 750 parts per thousand.