Reaper Prime was the first primed melee weapon, if the Skana Prime is disregarded.
Rubidium is an alkali metal element that is very soft, ductile and silvery-white in color.
Rubidium does not appear to be a very useful material for blades, as it is not hard enough to scratch other materials with its Brinell hardness of only 0.216MPa compared to Iron's rating of 490MPa. Being ductile, it is also easily bent, so it is unlikely to keep its shape when striking another object.
Furthermore, Rubidium, as an alkali metal, explodes when it comes into contact with water. It has also been known to spontaneously combust in air.