Agate is a term applied not to a distinct mineral species, but to an aggregate of various forms of silica, chiefly chalcedony.
Most agates occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles 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 of extreme tenuity, 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; thus a Brazilian geode lined with amethyst and weighing 35 tons was exhibited at the Dusseldorf Exhibition of 1902.
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 enclosing volcanic rock. This green silicate may give rise by alteration to a brown iron oxide (limonite), producing a rusty appearance on the outside of the agate-nodule. The outer surface of an agate, freed from its matrix, is often pitted and rough, apparently in consequence of the removal of the original coating. The first layer spread over the wall of the cavity has been called the "priming," and upon this base zeolitic minerals may be deposited.
Banded agate (agate-like onyx). The specimen is 2.5 cm (1 inch) wide.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.
On the disintegration of the matrix in which the agates are embedded, they are set free. The agates are extremely resistant to weathering and remain as nodules in the soil or are deposited as gravel in streams and shorelines.
Greek Agate is a name given to pale white to tan colored agate found in Sicily back to 400 B.C. The Greeks used it fo making jewelry and beads. Other than than it is today any agate of this colour or from Greek/Sicily area of the Mediteranean.
Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum due to the extreme delicacy of the successive bands, whence they are termed rainbow agates. Often agate coexists with layers or masses of opal, jasper or crystalline quartz due to ambient variations during the formation process.
Other forms of agate include carnelian agate (usually exhibiting reddish hues), Botswana agate, Ellensburg blue 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 to glow internally like an opal) and Mexican crazy-lace agate (which exhibits an often brightly colored, complex banded pattern).
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Jasper is an opaque, impure variety of quartz, usually red, yellow or brown in color. This mineral breaks with a smooth surface, and is used for ornamentation or as a gemstone. It can be highly polished and is used for vases, seals, and at one time for snuff boxes.
The name means "spotted stone", and is derived from Anglo-French jaspre, from Old French jaspe, from Latin iaspidem, the accusative of iaspis, from Greek iaspis, via a Semitic language (cf. Hebrew yashepheh, Akkadian yashupu), ultimately from Persian yashp.
The word yashepheh in the masoretic text of Exodus 28:20, referring to a stone in the Hoshen, is thus reflected in the Septuagint by the word Iaspis, and usually translated into English as Jasper. Despite the most common form of Jasper being red, scholars think that the yashepheh here actually refers to a green form of Jasper - which was very rare, and so highly prized; the greeks used Iaspis to refer to the green form, while the red form simply fell under the term Sard - which just means red. Rebbenu Bachya argues that this stone representing the tribe of Benjamin, but there is actually a wide range of views among traditional sources about which tribe the stone refers to.
It is described in the Book of Revelation (21:11) as follows: "It shone with the glory of God, and its brilliance was like that of a very precious jewel, like a jasper, clear as crystal."
Jasper can appear as an opaque rock of shades of red due to mineral impurities. Patterns can arise from the formation process and from flow patterns in the sediment or volcanic ash that was saturated with silica to form jasper, yielding bands or swirls in the rock.
Jasper may be permeated by dendritic minerals providing the appearance of vegetative growths. The jasper may have been fractured and/or distorted after formation, later rebonding into discontinuous patterns or filling with another material. Heat or environmental factors may have created surface rinds (such as varnish) or interior stresses leading to fracturing.
Egyptian jasper is a brown jasper, appearing as nodules in the Lybian desert and in the Nile valley between the Red Sea and Cairo, Egypt, distinguished by a zonal order of light and dark shades of colors. Egyptian jasper is also known as Egyptian pebble.