This monogram is from a 14th century Manuscript Alphabet book which resides in The British Museum in London, England. 

Monogram: S

Accents/Accessories: Smooth background; Small loose bail; Central monogram

Size: 27 x 48 millimeters (1.06 x 1.89 inches)

Weight: 7.60 grams (0.27 ounces)

Sterling Silver Chain as shown: 16 inches - $25.00 or 18 inches - $35.00