This is a dainty antique art nouveau baroque freshwater river pearl lavaliere pendant. 

The pearl is very baroque and wrinkled. It is probably from the Mississippi river which was a large freshwater pearl producer at the time. 

It is made of solid yellow gold and features an art nouveau design. It is set with a single blue stone that appears to be glass or paste. I measures 1 1/4 inch total. It is stamped "10K" on the bail. From the early 1900's

All pearls are truly vintage and have only been lightly cleaned.