Identify jewellery mark?

[dcam]

Hi,
I've recently found a ring in my parents loft. I'm pretty sure its not solid gold as its a really heavy mans signet type ring. The only marks inside are 18k and what looks like 725. I know silver is usually 925 but could this be a lower grade silver plated in 18k gold?
Thanks to anyone out there that can help.

[RATTER15]

Sterling silver

The standard alloy of sterling silver is composed of .925 parts pure silver and .725 parts pure copper. Mexican silver is alloyed with .950 parts silver and the remainder in copper. Prior to 1965, United States coin silver was .900 parts silver and the balance copper. Both types of silver are subject to oxidation when heated; however, in normal atmospheric conditions, the silver alloy reacts much sooner than the pure metal. The normal oxide occurring on the metal when heated is a sulfide film produced by the torch. Silver alloys also produce oxides, cupric and cuprous, formed by the presence of copper in the alloy. Tarnish on silver is produced by sulfuric oxides in the atmosphere and by certain foods or body salts..

as seen here http://www.handwovenbands.com/gold.htm

[Teddio]

Looks like 18 carat gold to me, is there a hallmark stamped on it? This alloy is 75% gold