There is a lot of information on the internet regarding the process by which rings are made today using modern tools. What techniques were used before the 17th century to do it?
By using soldering techniques to join the ends of the ring into one complete loop.
There is evidence that soldering was employed as early as 5,000 years ago in Mesopotamia. Soldering and brazing are thought to have originated very early in the history of metal-working, probably before 4000 BC. Sumerian swords from c. 3000 BC were assembled using hard soldering.
Soldering was historically used to make jewelry items, cooking ware and tools, as well as other uses such as in assembling stained glass.
So, the same techniques used to create stained glass, metal drinking/storage/votive vessels, etc., just smaller and more intricate.
For solid gold rings, an impure form of gold was used as a filler because it melts at a lower temperature than pure gold.
The higher the impurity of gold, the more quickly it will melt, and as such the impure gold would melt before the pure and could then be used to attach two or more pieces of purer gold