In Ancient Greece a Gorgoneion (or stone head, engraving, or drawing of a Gorgon face, often with snakes protruding wildly and the tongue sticking out between her fangs) frequently was used as an apotropaic symbol and placed on doors, walls, floors, coins, shields, breastplates, and tombstones in the hopes of warding off evil. In this regard Gorgoneia are similar to the sometimes grotesque faces on Chinese soldiers' shields, also used generally as an amulet, a protection against the evil eye.
source:http://en.wikipedia.org/wiki/Gorgon
Aegies, Spears, Helmets, Owl, Serpents, Olive trees, Armours and Gorgoneion.
It isn't always the same in Greek art... however, the most common is probably the gorgoneion (head of Medusa).
Turned them to stone if tthey saw it directly. If it was seen reflected in something, e.g. a shield, they were fine.
The words have different meaning, but they represent the same idea. An apotropaic sysmbol is something evil and ugly that scares away things that are evil and ugly. It frightens bad luck away. Gorogoneion "head of the gorgon" symbols on temples predate Medusa myths. The myth might have been written to explain the Gorgoneion carvings on buildings. Gargoyles are the same thing. Carvings of fantastic, imaginary, ugly, monstrous creatures that are believed to scare away other evil creatures. There are other theories of their purpose and origin. After the fall of Rome, everyone in Europe was trying to copy Ancient Greece and Rome, because they were so much more advanced compared to the dark ages.
Gorgon: In European bestiary, a strange beast of Lybia. It is described as having high, thick eyelids with eyes that look neither upward or forward. From its crown to its nose, it has a long, hanging mane. It eats deadly, poisonous herbs and if it is frightened, it will cause its mane to stand upright, opens its gaping mouth and sends forth a sharp, horrible breath that will infect the air above its head.ORA Gorgon was a monster from greek mythology, who had snakes for hair and fangs, and whose very glance could turn men to stone. According to Hesiod, there were three female Gorgons: Stheno, Euryale, and Medusa, the queen of them all. They were the Queens of the Underworld, and the one male Gorgon Nanas was Zeus' guardian. They were known as very evil and mean creatures. According to mythology, the blood from the right side of a Gorgon could raise the dead, but the blood from the left side was toxic. In ancient Greece, stone depictions of of a Gorgon head (gorgoneion) were often placed on doors, shields, graves, or wall to ward off evil.