a small stone or pendant worn like a necklace
Refers to pendants, gemstones that dangle from a neck chain.
A lavalier is a pendant with a dangling stone that hangs from a necklace. Lavaliers were named for the infamous Duchess Louise de La Valliere (1644-1710), a French woman who was a mistress of the French king Louis XIV (and was involved in many intrigues at court).
(Negligee Pendant): A necklace with two pendant s of unequal length suspended from it.