Plated or rolled gold over brass. I was in the jewelry bizz years ago and have seen this many times. It was common for crooked jewelers to use fake KT marks on plated or rolled brass. Legitimate pieces of gold plated or rolled gold were sold by legitimate jewelers but were stamped GF ( gold filled ), GP ( gold plate ) or RG ( rolled gold ). Very often these processes where used on pocket watches. Many times some one would come into my shop with the so called solid gold pocket watch passed down in the family asking what it is worth only to be very disappointed that it's plated with gold and worth a couple of hundred dollars in good condition and in working order. I have seen a few watches that where actually gold but they came from very wealthy families. Common people could never afford such a thing in the past or even today. Gold in a 24 KT state is a very soft metal and will wear quickly. Yellow gold is mixed with an alloy of brass. 24 KT being pure gold. 18 KT means 18 parts of gold mixed with 6 parts of brass, 14 KT means 14 parts gold to 10 parts of brass, 10 KT is 10 parts of gold to 14 parts of brass mixed as alloy. Price point is often a consideration when people select KT weight with 10 KT naturally being less expensive and the most durable because of it's brass content giving it strength.
Your ring that you found is very obviously plated with gold over brass and it's not an alloy. It having no other marks other then 18KT means the person that lost it was ripped off when they bought it. That's it for today's jewelry lesson.
