AGATE
Descpription:
Agate is a term applied not to a distinct mineral species, but to an aggregate of various forms of silica. It usually forms in concentric layers in a wide variety of colors and textures. The most common colours are white to grey, light blue, orange to red and black. Agate is often found as a round nodule, with concentric bands like the rings of a tree trunk. The first archeological findings of different implements made of agate date back to 8000 years ago.
There exist various types of agate. A Mexican agate, showing only a single eye, has received the name of "cyclops agate." Dendritic agates have beautiful fern like patterns on them formed due to the presence of manganese and iron ions. Other types of included matter deposited during agate-building include sagenitic growths (radial mineral crystals) and chunks of entrapped detritus (such as sand, ash, or mud). Occasionally agate fills a void left by decomposed vegatative material such as a tree limb or root and is called limb cast agate due to its appearance. Turritella agate is formed from fossil Turritella shells silicified in a chalcedony base. Similarly, coral, petrified wood and other organic remains or porous rocks can also become agatized. Agatized coral is often referred to as Petoskey agate or stone.
Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum due, hence they are termed rainbow agates. Often agate coexists with layers or masses of opal, jasper or crystaline quartz due to ambient variations during the formation process.
Other forms of agate include carnelian agate (usually exhibiting redish hues), Botswana agate, blue lace agate, plume agate (such as Carey, Graveyard Point, Sage, St. Johns, Teeter Ranch and others), tube agate (with visible flow channels), fortification agate (which exhibit little or no layered structure), fire agate (which seems glow internally like an opal) and Mexican crazy-lace agate (which exhibits an often brightly colored, complex banded pattern).
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. Agate bowls were popular in the Byzantine Empire. Collecting agate bowls became common among European royalty during the Renaissance and many museums in Europe, including the Louvre, have spectacular examples.
History:
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 other gemstones.
Most agates occur as nodules in eruptive rocks or ancient lavas where they represent cavities originally produced by the disengagement of vapour in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Such agates, when cut transversely, exhibit a succession of parallel lines, often extremely tenuous, giving a banded appearance to the section. Such stones are known as banded agate, riband agate and striped agate.
In the formation of an ordinary agate, it is probable that waters containing silica in solution -- derived, perhaps, from the decomposition of some of the silicates in the lava itself -- percolated through the rock and deposited a siliceous coating on the interior of the vapour-vesicles. Variations in the character of the solution or in the conditions of deposit may cause corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline quartz. Several vapour-vesicles may unite while the rock is viscous, and thus form a large cavity which may become the home of an agate of exceptional size.
The first deposit on the wall of a cavity, forming the "skin" of the agate, is generally a dark greenish mineral substance, like celadonite, delessite or 'green earth, which are rich in iron probably derived from the decomposition of the augite in the mother-rock. This green silicate may give rise to a brown oxide of iron (limonite), producing a rusty appearance on the outside of the agate-nodule. The outer surface of an agate is often pitted and rough, apparently in consequence of the removal of the original coating.
Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, often amethyst, having the apices of the crystals directed towards the free space so as to form a crystal-lined cavity, or geode.
Being a siliceous material, which is extremely resistant to the action of air and water, agates remain as nodules in the soil and gravel, or become rolled as pebbles in streams.
Characteristics:
Chemistry: Silica, SiO2
Hardness (Mohs Scale): 7
Density: 2.6
Refractive index: 1.53 – 1.54
Dispersion: none
Crystal habit: cryptocrystalline silica
Crystal system: hexagonal
Cleavage: None
Luster: Waxy
Transparency: translucent to transparent
