Not unlike what we have seen on the Antiques Roadshow Public Broadcasting program, dating jewelry is very much like assessing when a piece of furniture was made. All the elements of the item of jewelry are examined and put into the context of the history of jewelry.
With the specific example of this ring several features that stand out. First, the deep wells of gold which hold the stones are used for a foiled and closed back setting. Then there are the gold work cusps that are shaped around the exterior of the center well which are indicative of work done around the late 17th and early 18th centuries.
The cut of the diamonds is next with one which is a table cut – a very early cut of diamond along with two rose cuts. Rose cuts were also in favor around this time period and even used until today but the cut was the preferred one (along with old mine cuts) in the 18th and 19th centuries.
Another aspect deals with the underside of the setting. Under the center well there is a deeply worked very specific ridged "sun ray" pattern which is often indicative of a ring from this period. Finally, the gold openwork bridges underneath at the sides as well as the small upturned connection points from the shank to the outer diamonds and gold work are an indicator of this era. While many other factors must be considered, the ones mentioned are excellent clues as to extensive age and mysterious history from long ago.
The ring itself is 18k yellow gold and features a trio of diamonds (a center rose cut of 4 mm across, 1 table cut and one rose cut) set horizontally in the previously stated deep cusps or wells. The simple scallop engraving along the center cusp is typical for the time as is the modified floral forms along the shank.
Important: Foiled jewelry should not be exposed to water for any extended period of time nor soaked in any other liquids, cleaning products or harsh chemicals.. In addition, avoid steam cleaning. If considering as an engagement ring, please keep this in mind.