Based on the thickness I expect it was made to hold hair. The stone is onyx, and the gold was tested as 15k. It is in nice condition, the only issue being that one of the glass sections (the right one) tends to open a bit when you open the locket. It is close to 14 grams in weight, and I'd guess that the gold would be able 6 grams of that weight.
Please ask me any questions you might have.