Archaeologists have found evidence that kits for grinding malachite eyepaint were a very important item in the Egyptians' toilette. Interestingly, the Egyptian word for these kits is quite similar to the Egyptian phrase "to protect." Wearing Udju was practical in reducing the glare of the sun (much as football players wear grease paint), and was believed to guard against the evil eye. However, it's unlikely that the ancient Egyptian women realized that the metallic ions of malachite (an oxide of copper) made an extremely efficient antibacterial agent, protecting them from all types of infection. Infections of the eyes were common due to flies and flying sand, and wearing malachite likely reduced them.
However, long-term use of Udju couldn't have been healthy -- the high copper content of malachite (57%) makes it toxic. This makes it dangerous to mine, cut, or hand-polish malachite without proper protective equipment. Luckily, it's completely safe to handle rough and polished specimens.
Malachite is a secondary mineral of copper, which means it's formed when copper minerals are altered by other minerals. It occurs when carbonated water interacts with copper minerals, or when a solution of copper interacts with limestone. Malachite forms relatively quickly -- bronze objects have been found at ancient Assyrian sites partially or wholly converted into malachite! The green patina on weathered copper roofs and statues is a form of malachite. Because of its presence in nearly all oxidized copper deposits, malachite serves as a prospecting guide for (and an important ore of) copper, and is found in small quantities all over the world.
The distinctive banding is caused by subtle changes in the oxidation states of the surrounding pore waters, but the exact mechanism is still not well understood. Though the massive form is the most well-known, having been used for carved figures and household decorations for centuries, a less common but spectacular form is fine acicular hairs that group together to form a carpet of green "velvet," as shown here.
For metaphysical purposes, Malachite is thought by many to be useful in balancing emotional states.
Malachite: Cu2(CO3)(OH)2, Copper Carbonate Hydroxide
Habit: Massive forms are botryoidal, stalactitic, or globular; crystals are acicular or fibrous and form in tufts and encrustations.
Mohs scale hardness: 3.5-4
Luster: Massive forms are dull; crystals are silky.
Transparency: Massive forms are opaque; crystals can be translucent.
Cleavage: Perfect, but rarely seen.
Fracture: Conchoidal to splintery
Specific gravity: 3.9+ (noticeably heavy)
Other: Weakly effervesces in acid.