The last time I walked through the Hall of Gems at the American Museum of Natural History, three particular opals caught my eye. Their shape was familiar but I couldn't place it, so I looked closer, and discovered that they were fossilized clams. After I snapped a picture, I went home to do a bit of research.
According to the Australian Opal Center:
Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried.
In 1987, a miner found a near-complete opalized pliosaur fossil, which is in the collection of the Australian Museum. The Addyman Plesiosaur, another near-complete opalized fossil, is on display at the South Australian Museum. There don't seem to be any great photos of either specimen online, but just knowing that such a thing exists is amazing. Just imagine: a glimmering, iridescent, meters-long opal skeleton.
Not to mention opal pine cones and sea shells.
More opal fossils to look at here.