Uranium glass is a type of glass that has had uranium added to it before melting to create colors. The proportion of uranium in the glass usually varies from trace levels to about 2% by weight, although some pieces have been found to contain up to 25% uranium. The addition of uranium gives the glass a characteristic fluorescent effect, causing it to glow bright green under ultraviolet light. This fluorescence is one of the most distinctive features of uranium glass. Despite being mildly radioactive, the level of radioactivity in uranium glass is very low, and it is considered to be safe to handle. Most pieces of uranium glass are now considered antiques or retro-era collectibles, and modern production is mainly limited to small objects like beads or marbles for scientific or decorative purposes