Very nice collar choker style necklace marked Mexico Taxco 925. The collar is about 14 inches in total length. Entire circumference was about 18 inches but can be adjusted. It is 2.3 mm thick. 

Comes with a very nice Ammonite shell pendant which is rather large and dyed red. Pendant is encased in sterling silver casing. Pendant measures 2 inches long by 1.5 inches wide.