The 'Husk' free-flow sippy cup (by OL Pro) is made out of rice husks - an agricultural waste product. The husks (which surround the rice grains) are collected, crushed up and bonded with plant-based resin. It feels just like plastic.
These 250ml cups (costing between £5 - £10) are suitable for toddlers aged 18 months and over, as it only has one handle, the lid can be rather easily removed and the spout is a free flow design (meaning a probable flood if younger children were using it). It is very successfully used as a night-water-by-the-bed-cup in our household. The Husk cup would also suit adults who need a lidded cup to drink from, as the simple design is neutral / utilitarian.
There is a choice of lid colours - blue, green, orange and pink. The lid is made of food-grade silicone, but it does need a good daily scrub to keep it clean (a 'quick rinse' won't do) as the lid dips down towards the spout, meaning the surface stays wet and can get grotty. Luckily it can go in the dishwasher. Reviews say that microwaving it makes the cup hot while the contents remains cold, but I haven't used it in the microwave so I haven't had that issue.
I am glad to have found a sippy cup which, wherever it ends up (toddler cups have a habit of disappearing when out and about), will predominantly biodegrade.