Many pieces of opaline glass are decorated with gilding. Opaline silica and iron sulphate minerals form in acidic (low pH) solutions. Real opaline glass was produced only in France. Another surviving example of Gray's work is an etched opaline panel showing a fern gully. Opaline silica is often deposited in hot springs. The Opaline Theatre, The Palazzo Theatre, The Sands Showroom, and The Venetian Theatre. All opaline glass is hand-blown and has a rough or polished pontil on the bottom. The term "opaline" in current times refers to many forms of opaque and colored glass. In the 20th century Italy produced a similar glass and called it opaline veritable. Most opaline glass is not branded or signed. The popularity of Opaline glass began during the reign of Napoleon. In the Gusev crater of Mars, the Mars Exploration Rover Spirit inadvertently discovered opaline silica. Magnessium-rich clay minerals and opaline silica have been detected in the area. The idea that the term opaline is strictly antique French crystal is incorrect. The presence of opaline silica points toward a hydrothermal environment that could support life.