--- Minerals list
--- Silicates index
|From "achates" (Greek) said to be first found near the river Achates in Scily
There are a wide range of colours and patterns. See this page for some examples.
Agate occurs typically as a cavity filling in lava. This means that there is a bubble of gas in the hot lava. When the lava hardens, this bubble fills up with silicates and this is called a geode. You can buy complete geodes, which look very dull from the outside, but when opened glisten with quartz crystals.
You can also buy slices of agate, which show the patterns to their best effect. When the geode fills up, it may harden in layers, with different colours. Sometimes there is still a gap in the middle, which may then grow quartz crystals.
Be careful when you buy agate. It is easy to dye this stone, and the more gaudy colours that you see are not natural colours. Sometimes you can even see traces of the dye on the cracks on the outside of the geode!
There are other types of agate. See Other Chalcedony.
Larger picture of Agate: