Agate

Agates are a microcrystalline (microcrystalline means that some of the crystals can only be seen under a microscope) form of silica. They have very fine grain, and generally have bright colours.

The stones were given their name by Theophrastus, a Greek philosopher and naturalist, when he found it along the bank of the river Achates.

Agates were used in ancient times for hard-stone carving. Hard-stone carved things have been found all over the ancient world. One of the places it was found is Knossos, Crete.

Agates are often found in clumps in volcanic rocks and ancient lava. They form when gas bubbles trapped in solidifying lava are filled with silica and alkali-bearing waters, which thicken into a gel. The alkali attacks the iron in the surrounding rock and lava, and bands of iron hydroxide are made in the gel. The gel crystallizes, and leaves the bands intact. This makes the many bands we see in jewellery today.

There are many different types of agates. The Mexican Agate, showing only one “eye”, is called a cyclops agate. Turritella agate is formed from fossilized shells. Coral, petrified wood, and other 'organic' materials can also become agatized. Greek agate is a name given to light-coloured agates found in Sicily and other ancient world places.

Agates can be found around the world. One of the most famous places is Brazil, where very valuable agate is found. Western United States, Mexico, Germany, Italy, China, India, and Africa are some of the many other places they can be found.

http://www.rps.psu.edu/0109/form.html
http://answers.yahoo.com/question/index?qid=20081221182519AAyhtmE
http://www.gemstoneeducation.com/Agate.htm
http://en.wikipedia.org/wiki/Agate