-Quartz type of banded chalcedony (SiO₂), it is one of the most widely distributed gemstones, and it receives universal acclaim because of its wide variety of form, size and color.  Agates occur abundantly in vesicular volcanic rocks, and theories as to their mode of formation as too numerous to be summarized here.

Although discussions of the genesis of Agate date at least to the end of the 18th century (eg Bruckmann, 1773), most publications are only of a descriptive nature.  Over the last two decades, however, new mineralogical and geo-chemical data have already advanced our knowledge of the mineralogy and genesis of Agate.

Nevertheless, the origin of Agate remain incompletely understood and discussions are controversial especially due to the fact that no one has unambiguously documented Agate formation in real time and Agates have never been successfully replicated in the laboratory.

Some hot springs in areas with geothermal activities or white smokers on the ocean floor are depositing something like chalcedony (Hopkinson rt al., 1998) but they are not producing Agate...

Analyses of the distribution of trace elements and stable isotopes should provide important information concerning the geo-chemistry of Agate and genetic aspect of Agate formation.  Because of he widespread occurrence of Agate in certain volcanic rocks, the aim would be to evaluate similarities and differences in the geo-chemical characteristics between volcanic Agate samples from various locations and different parent rocks.

The geo-chemical investigation of Agates, their parent volcanic rocks and associate calcite mineralisation represent an extensive analytical characterization of Agates from localities around the world.  Despite the different origin of the Agate samples investigated, some common geo-chemical characteristics were observed.

The gemstone:
No gemstone is more creatively striped by nature than agate, chalcedony quartz that forms in concentric layers in a wide variety of colors and textures. Each individual agate forms by filling a cavity in host rock. As a result, agate often is found as a round nodule, with concentric bands like the rings of a tree trunk. The bands sometimes look like eyes, sometimes fanciful scallops, or even a landscape with dendrite trees. 

Agate: Background
Agate was highly valued as a talisman or amulet in ancient times. It was said to quench thirst and protect from fevers. Persian magicians used agate to divert storms. A famous collection of two to four thousand agate bowls which was accumulated by Mithridates, king of Pontus, shows the enthusiasm with which agate was regarded. Agate bowls were also popular in the Byzantine Empire. Collecting agate bowls became common among European royalty during the Renaissance and many museums in Europe, including 'Le Louvre', have spectacular examples. 

The mining of agate in the Nahe River valley in Germany which was already documented in 1497 gave rise to the cutting center of Idar-Oberstein, Germany. Originally, the river was used to power the grinding wheels. When the Nahe agate deposit was exhausted in the nineteenth century, Idar cutters started to develop the agate deposits of Brazil, which also sparked exploration and discovery of Brazil's rich deposits of amethyst, citrine, tourmaline, topaz, and other gemstones. Although the small town of Idar-Oberstein is still known for the finest agate carving in the world, today Idar imports a huge range of other gem materials from around the world for cutting and carving in Germany and Asia. Cameo master carvers and modern lapidary artists flourish along with rough dealers who scour the world for the latest gem discoveries for export. And the entire industry sprung from the taste for agate bowls and ornaments during the Renaissance! Maybe agate is also a powerful talisman for success in international trade? 

Agates and Jaspers, think Crypto 
Agates and Jaspers are a crypto-crystalline (crystals too small to be seen even by a microscope) variety of quartz, known as chalcedony. The varieties of chalcedony are divided more by character than by color. Chalcedony is divided into a host of gemstones that have been named for locally common varieties. Some of the more beautiful types have retained their names on a world-wide basis while other names have faded into obscurity as many of the relatively small local deposits became exhausted. Some of the more common of these types are chrysoprase (a pure green agate), sard (a yellow to brown agate), sardonyx (banded sard), onyx (black and white agate), carnelian (a yellow to orange agate), flint (a colorful and microscopically fibrous jasper), jasper (a colorful opaque agate) and bloodstone (a green with red speckled jasper). 

Chalcedonies
Quartz that is formed not of 1 single crystal but finely grained micro-crystals is known as chalcedony.  The variety of chalcedony is even greater than transparent quartz varieties because it includes crypto-crystalline quartz with patterns as well as a wide range of solid colors.  Agates are banded, bloodstone has red spots on a green ground, moss agate has a vegetal pattern.  Japser sometimes look like a landscape painting.  Another staple of of the jewelry industry is black onyx, chalcedony quartz which owes its even black color to an ancient dyeing process that is still used today.  Carnelian, another chalcedony valued in ancient world, has a vivid brownish orange color and clear translucency that makes it popular for signet rings and seals.
Chrysoprase, a bright apple green translucent chalcedony, is the most valued.  It was a particular favorite of Frederick the Great of Prussia, who loved its bright green color.  It can be seen today decorating many buildings in beautiful Prague, including the Chapel of Saint Wenceslas.  Chrysoprase is found today mostly in Australia.  Unlike most of other green stones, which owe their color to vanadium or chromium, chrysoprase derived its color from nickel.  Its bright even color and texture lends itself well to beads, cabochons and carvings.