Three Rivers is the name of a city in Michigan, it takes the singular because it is the town which has three rivers.
If we wrote the following
The three rivers are in southwestern Michigan.
the meaning changes. Note the lack of uppercase letters in "three rivers", using lowercase letters and the plural verb avoids confusion.
Using a cardinal number with a common noun such as the OP's nine stones, and three rivers, we have another real-life example
Seven Sisters is a famous road and underground station in North London.
However, the plural verb is used when referring to the cliffs in Sussex, England.
The Seven Sisters are a series of chalk cliffs by the English Channel. (Wikipedia)
The Seven Sisters Cliffs in England are quietly famous. They’re the tallest chalk cliffs in England and one of the most spectacular coastlines in the whole of the UK. (Passport Collective)
The plural noun is also retained for the Norwegian mountain range Seven Sisters which according to legend were seven troll sisters turned into stone by the morning sun.
The Seven Sisters are all so close that a good hiker can climb them all in a single day. The Norwegian Trekking Association estimates that a fit walker should take just under thirteen hours to complete the 27 kilometre hike,… (Hurtigruten)