Terminology

An agate (AG-it) is a distinctly-banded, fibrous type of chalcedony which forms in various rocks. Agate is made of length-fast chalcedony fibers, sometimes with layers of length-slow chalcedony fibres (Michel-Lévy and Munier-Chalmas 1892; Correns and Nagelschmidt, 1933; Bernauer 1927; Braitsch 1957; Frondel, 1978; Flörke et al. 1991; Graetsch 1994). Two characteristic types of banding can be distinguished in agates:

1. Wall-lining Banding. The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibres. Since the chalcedony fibres grow from the walls to the interior of a cavity, a concentric pattern develops. The changes in translucency reflect periodic changes of crystallite sizes and repetitive nucleation of new fibres at the growth front (Lu and Sunagawa 1994; Cady et al. 1998), as well as chemical composition (Frondel 1978; Heaney and Davis 1995). In addition to the visible bands, there are compositional bands of varying trace element and hydroxyl concentrations on the micrometer scale (Frondel 1978, 1985). 

2. Horizontal Banding. This type of banding is less common and usually accompanied by wall-lining banding. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitate in the cavity. Horizontal bands can serve as “spirit levels” to determine the original orientation of the specimen in the host rock.

 A botryoidal (BOT-ree-OY-dəl) texture or mineral habit, is one in which the mineral has an external form composed of many rounded segments, named for the Ancient Greek βότρυς (bótrus), meaning "a bunch of grapes". Minerals take on a botryoidal habit when they form in an environment containing many nuclei, specks of sand, dust, or other particulate matter to serve as sources of crystal nucleation. Fibrous crystals grow outward from these "seeds" at the same or very similar rate, resulting in radial crystal growth. As these spheres grow, they can run into or overlap with others that are nearby, fusing together to form a botryoidal cluster. 

CHALCEDONY - Depending on the context, the term "chalcedony" (kal-SED-ə-nee) has different meanings.

1. As a more general term for all varieties of quartz that are made of microscopic or submicroscopic crystals, the so-called microcrystalline and cryptocrystalline varieties of quartz. Examples are the different types of agate, jasper, chert, chrysoprase, onyx, pietersite, etc.

2. In the strict sense, and in the scientific literature, "chalcedony" designates aggregates of parallelly grown ("fibrous") quartz crystals of microscopic and sub-microscopic size. Based on the conspicuous behaviour of thin sections of chalcedony in polarized light, at least two types can be distinguished (Michel-Lévy and Munier-Chalmas, 1892; Correns and Nagelschmidt, 1933; Braitsch, 1957; Frondel, 1978; Flörke et al. 1991):

   • length-fast chalcedony, with crystallites stacked perpendicular to the c-axis, and the resulting fibers being elongated either along [1120] or -more rarely - along [1010]. The fibers may be twisted around the elongation axis.

   • length-slow chalcedony or Quartzine, with crystallites stacked parallel to the c-axis, and the resulting fibers being elongated along [0001], like in macrocrystalline quartz.

Both types tend to develop radially grown "fibers", resulting in botryoidal, rounded and stalactitic habits. They often show concentric banding perpendicular to the fiber orientation and are then called agates.

Strictly speaking, most chalcedony is a mixture of cryptocrystalline quartz and moganite. In common practice, only the translucent, single-color types of this material are sold under the name “chalcedony,” and the rest of this group are sold under individual variety names or as agate or jasper.

Cryptocrystalline is a rock texture made up of such tiny crystals that its crystalline nature is only vaguely revealed even microscopically. “Microcrystalline” is a related term similarly used to describe a rock or mineral with crystals too small to be seen with the naked eye—and although microscopic, microcrystalline crystals can usually be seen when magnified. “Cryptocrystalline” specifically describes a rock or mineral whose crystals under magnification remain so tiny to the human eye that they look like a dense, compact and uniform mass.

Historically, the name has long been used for an opaque, solid or patterned impure chalcedony or cryptocrystalline chert, usually containing abundant inclusions of hematite, iron hydroxides and other minerals. It is typically associated with banded iron formations. Jaspers, in general, are very common; hence most of the value in a given piece relates to the saturation of its color, the beauty of its pattern or the artistry with which it is fashioned. 

Although the term “jasper” is now restricted to opaque quartz, the ancient “iaspis” was a stone of considerable translucency. The jasper of antiquity was in many cases distinctly green, for it is often compared in contemporary records to emerald and other green objects. The jasper of the ancients certainly included stones which would now be classed as chalcedony, and the emerald-like jasper may have been akin to the modern chrysoprase. 

Green jasper was used to make bow drills in Mehrgarh between 4th and 5th millennium BC. Jasper is known to have been a favorite gem in the ancient world; its name can be traced back in Arabic, Persian, Hebrew, Assyrian, Greek,  and Latin.