Agate has to be one of the most frequently mis-labeled minerals out there. There are so many varieties, and many have several names, yet they all have enough in common to be considered agate. The correct definition is a microcrystalline (crystals so small they cannot be seen with the naked eye), fibrous, variegated chalcedony, often banded. In other words, all agate is chalcedony, but not all chalcedony is agate. In general use, if it is more or less "plain" it is known simply as chalcedony, unless it has a special name for its colour, such as carnelian for the pale orange through deep red range.
In turn, chalcedony is a quartz, and agate is therefore a silicon mineral. Chemically, agate is simply SiO2, or silicon dioxide, as all types of quartz are, but differences in the way layers of the mineral have formed over time, as well as inclusions of other minerals, allow for an endless variety of patterns and colours.
A specific type of agate, and commonly confused with it is onyx, in which the bands are parallel and fairly regular, and the crystals are even smaller, known as cryptocrystalline. Because onyx is highly valued, it is not unusual for banded agates to be labelled as onyx, and indeed even unrelated minerals to be passed off as onyx.
The other mineral commonly confused with agate is its close cousin jasper. This is another microcrystalline chalcedony, the key difference being that jasper is opaque. While larger pieces of agate frequently appear to be opaque, at a crystal level they are in fact translucent. However these terms are not always strictly adhered to, and there is some, well, let's say wiggle room with nomenclature, even among experts.
Agate is relatively porous, and for this reason is often dyed, sometimes in lurid neon colours. Large decorative coloured pieces are quite reasonably priced and popular in Feng Shui. In the jewelry trade agates are commonly enhanced with dye to make an endless variety of patterned beads, as well as being used in their natural state, depending on individual tastes. It is a very economical stone to use, being plentiful, as well as being strong yet easily shaped, and is therefore incredibly popular.
Some of the better known named varieties such as Botswana agate (grey/brown bands), moss and tree agates (green clouds or branches) and crazy lace agate (multi-coloured, often with bright brick red, and sometimes dyed to create blues) can fetch a high price for good quality specimens. Petrified or fossil wood is also an agate, all the organics having been replaced by quartz.
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