"Gems" is a collection of "stamps" - fully onchain tokens that encode their image data directly within their token URLs. Instead of external image hosting, "stamps" use smart contracts to output images you can render in any standard web browser.

The gems are rendered in a 3D rendering and lighting system that lives entirely in Solidity smart contracts. Each gem is uniquely generated from a single onchain seed.

Gems on OpenSea
More on "stamps"