This jar contains a 99.7% pure sample of element 51 on the periodic table: antimony (Sb).
Most chemists classify antimony as a metalloid, since it has some metallic physical properties, like luster and electrical conductivity, while most of its chemical properties are similar to non-metals.
It was first defined in 1540, but compounds of antimony have been known and used since about 3,100 BCE by Egyptians. The origin of the name antimony has been lost to history. There are several theories, but each include uncertainties.
Many materials can be made fireproof with the addition of antimony compounds. And when alloyed, antimony can raise the hardness and tensile strength of metal objects. However, antimony is soft and not an ideal pure material to make objects with. China briefly minted an antimony coin, but they were discontinued due to poor durability.
Antimony occurs in 4 forms, called allotropes, including one that reacts so violently to the slightest disturbance it is simply called explosive antimony. This sample is non-explosive, non-toxic, and stable (non-radioactive).