Cobalt blue, the intense deep jewel color of sapphires, is one of the rarest colors of sea glass that you still have a chance of collecting if you keep at it long enough. It is a more expensive color to manufacture as it includes the metal cobalt into the mix. Vintage cosmetics and home remedies were bottled in cobalt blue including: Ponds, Noxema, Vicks, and Milk of Magnesia.
Antique poison bottles were also bottled in cobalt blue glass. The dark glass helped to preserve the contents by shielding them from sunlight.