Treehugger points to these tiaras made from supermarket plastic bags by Glovedup in London.
From the site:
If you need a tiara for your next eco ball or costume party, here’s the answer. These tiaras are modeled after historical Russian and English designs and modernised for today’s glamorous world. They are made out of red, purple or red Waitrose supermarket plastic bags collected by the designer’s sister who gets them from home deliveries. Then they are baked and moulded into shapes. Some are crotcheted into little evening bags to match the tiara, or knitted into tam’s.