The "NR" stands for the designers Nancy & Rise Ltd., New York 1987.
The husband-and-wife team Nancy Schelsinger & Rise Egert made the more high end costume jewelry which has become quite collectible with collectors.
They were bought out in 1997 by Frank Jedda & Ezra Sheena of Frank Stella.
Page with the trademark information:
I see no other markings as to the type of metal or stone..
The clear, crystal stone is about 3/8" by 5/8" (about 14mm by 8mm)
It has a ring guard so it can be worn from about a size 8 to a fairly tight 9.
It looks in good vintage shape, could use a cleaning.
*All items are "as is" and there are no returns so please read the descriptions and see the photos before you buy.