a pendant with a lustrous blue gem

Over a hundred small rings have been linked together form a chained necklace with a small hook clasp on one side, and a pendant on the other end. On examining the chain further, it appears to be quite sturdy despite its rather diminuitive size. The pendant is shaped like a teardrop and has room for a gem to be mounted. The pendant has been embedded with a lustrous opal.

It is a pendant with a lustrous blue gem.
It weighs half a pound.

You identify a pendant with a lustrous blue gem.
It is an anti-summoning object. It is efficient against long range spells.
It weighs eight ounces, and it is worth about eleven gold.
Its keywords are pendant, lustrousblue-pendant, lustrous, and blue.