Sodium

Many things can give you sodium, and it has a chance of dropping from decomposing cobblestone.

Occurrence and Properties
Lapis lazuli is an abundant stone found as an ore. When lapis lazuli, in its pure state while excluding the ore, will produce the chemical, lazurite. Lazurite will decompose into eight vials of elemental sodium. Sodium acts the same as a solid element, however it is more reactive than the standard metal. When a vial of pure sodium is used over a water block, a large column of gas and explosion will result. This is a reference to sodium's reactive nature when exposed to water.