Plastic used to be made of milk, not oil.

• From the early 1900s until about 1945, milk was commonly used to make many different plastic ornaments, including buttons, decorative buckles, beads and other jewellery, fountain pens, hand-held mirrors, comb and brush sets. Combining the liquid with formaldehyde.
• Milk plastic (usually called casein plastic) was used for jewellery by Queen Mary of England. Lactoid was a popular brand back then. Here's a few interesting reads into the history of plastics - ScienceMuseum, ScientificAmerican, V&A