Ben's and my friend Tove has a great blog, Thread for Thought, about the history of fashion. I was particularly moved and interested by her post about mourning:
Queen Victoria’s obsession with the public mourning of Prince Albert resulted in a great demand for fashionable and affordable black jewelry, and jet became a popular material for jewelry and buttons. It is an incredibly dense, dark mineraloid derived from decaying wood, appropriately enough. It has been imbued with a religious significance too, as it is a traditional material for monks’ rosaries. Queen Victoria sported and popularized Whitby jet, which initially created a boom in the industry but hampered its long term usage as people associated the stone with death. Vulcanite was another material of similar properties commonly used for mourning jewelry.