The subtle sheen Lydia makes this a uniquely beautiful tumbler. Lydia rock, also known as volcanic glass, is formed as molten lava cools. The result is a dense, glossy rock with subtle banding and luster. This luxe tumbler is crafted of beautifully nuanced, deep Lydia rock - also known as volcanic glass. The burled patterns and lustrous effects occur as molten lava cools and transforms into a dense, glossy rock.