Lapus head dress

Vivien Leigh looks absolutely beautiful in this film. Her costumes are wonderful. I especially love the one that appears to be made of blue lapus lazuli stones. She wears it near the end but not at the very end.

Does anyone know anything about it? It seems to me that if it was really made of lapus stones it would be uncomfortably heavy and it doesn't appear to be heavy. It doesn't look fake as if it were made of paper mache. Are there any costume experts out there?