Uranium Glaze.

Ceramic glaze is a shiny coating applied to ceramics before firing. Uranium glaze is a type of ceramic glaze that contains uranium creating various colours and can emit a special glow under UV light. The radioactivity in uranium glazes is generally considered safe.

They were popular in the early to mid-20th century but are less common now. Antique ceramics with uranium glazes are sought after by collectors for their historical value and unique appearance.