I bought a bag of childrens plastic toy rings from a supermarket for £1 and used one of those to propose so she could choose the ring she wanted.
In the end we had rings made, her engagement ring is a diamond solitaire, her wedding ring is encrusted with black diamonds and the two interlock to make a single larger ring. My wedding ring was made too, simple band but with a subtle nod to the shape of the mobius strip. Worked out around 10% more expensive than buying the equivalent size/gems/weight in a high street jewellers, but they are unique.