Rubidium is a rare and expensive metal with few uses. Ironically, its rarity in part is due to there being few recipes that mark Rb as an important ingredient. It have a solitary extra electron in its outer shell means it will hungrily pair up with almost any other element and form an especially strong bond. This explains why it is expensive on top of being rare: a number of tiresome processes are required to break that chemical bond and leave the metal isolated in its pure state.

The collector will note that rubidium is probably the most difficult of the naturally occurring and stable elements to acquire. And gram for gram it is much more expensive than gold - at least as prepared for lab use such as in this tiny ampule containing about 100 milligrams. 

Very shiny and handsome, the element tends toward an off-white creaminess and is extremely soft... a characteristic not immediately apparent since freeing it from the glass will immediately invite a violent reaction as the metal quickly grabs on to just about any material it comes into contact with including, of course, the oxygen in the air.