Encyclopedia
Rubidium
Silver-white solid substance (specific gravity 1.5), melts more easily than sodium. It is stored in sealed ampoules or in mineral oil
Source: "Explanatory Notes to the Harmonized System of the Customs Tariff of the Russian Federation (HS Russia)" (prepared by the Federal Customs Service of Russia) (Volume 1, Sections I - VI, Groups 1 - 29)