The pendant measures approximately 1 and 9/16 inches high, including the bale, by about 13/16 of an inch, or about 20.6375 millimeters, wide, and about 5/16 of an inch thick. The bale opening is just over about 1/8 of an inch, or approximately 3.5 millimeters, wide. (Included is a US Quarter in the photographs, for the measurement-challenged and ruler-averse.) It is stamped, "925", on the back, unsigned.
The chain measures about 20 and 1/2 inches long, from end to end, and will fit, closed, at between 18 and 1/8 inches and 20 and 1/8 inches in length, depending on which link you choose to catch. The square chain is approximately 2 millimeters, or just over about 1/16 of an inch, wide on a side. Stamped on both ends, and again on the spring ring clasp, "925", unsigned. Combined weight is approximately 15.4 grams, which includes the stone and shell weight, of course.
This necklace is in very good vintage condition. No visible damage to the stones or the shell. The chain has one or two minor bends, likely from the heavy pendant resting on it. The clasp works well and holds securely. Hand polished, but may have some remaining tarnish, especially in the crevices around the hinge on the two sections of the pendant.
A very beautiful, bold but feminine, vintage Sterling silver, garnet, and MOP pendant necklace. Heavy and chunky, while feminine and sparkly. Lovely detail on the pendant. A standout piece!