My first K18 find as I understand from some googling it stands for a mixture of 14K and 18K. Anyway clearly gold and a heavy one too so one happy chappy here
As I examined the ring more closely the face plate is lighter colour than the body so I would assume the face is made of 14K and the body 18K but who knows..
K18 is a first for me. I tried looking it up on the internet and it seems to be a designation for 18K by Japanese jewelry makers. I didn't come up with any case where they talked about mixed metals, except when I saw a ring listed as K18/Pt 950 which is the typical designation for a mixed metal. Any which way you slice it, it is a very nice find....