The ancients believed that hematite was that shield. It is said to have been used for "magic mirrors" that could reflect negativity back to where came. Early Romans would crush the mineral and rub its red powder onto their bodies, believing this would bestow courage, strength, and a general invulnerability.
Nowadays, it's worn mostly for mental healing & grounding, protection from negativity, and promoting physical health.
Hematite rings are very common, and will absorb and hold your stress. However, when your little shield has taken as much as it can hold, it will break. So far, I've only broken one but I lose mine all the time (is that sign?). My friend Allison is constantly replacing her broken rings.
The great thing about hematite - other than its magical properties - is the coloring: a perfect neutral, it goes with everything. Gray has become the new black so it's perfect for pairing with classic or current trends, like gold and the military.