Glossary

Agate banding.
The most visible banding in agates is the coloured banding due to the segregation of its pigmenting impurities, for example iron oxide producing reddish tints. But this banding is not as fundamental to agate as is the systematic growth banding associated with the spherulitic crystallization of chalcedony in the clear chalcedony layer and the relatively longer fibrous growths of chalcedony in the rest of the agate.
Agate dyke.
In this book an agate dyke is the name given to a minutely raised lineation that commonly occurs on the exterior of agates. It is the surface expression of a rent in the clear chalcedony layer through which a small amount of nascent agate has been expelled from within some agates. An agate dyke is not usually seen on abraded agates or on agates with thick skins. Dykes can be obvious but are usually subtle features best seen with a binocular microscope and illuminated with sidelighting. Once recognized they are quickly discernible in other agates. They may sometimes run nearly all the way round an agate from near one tube of escape to another.
Agate names.
There is no agreed way of spelling agate names. Some names are hyphenated, others are not. All agate names in this book are written without a hyphen.
- Brecciated agate: agate which has been broken into angular fragments and later recemented by silica solutions.
- Carnelian agate: Agate consisting, in whole or part, of carnelian - which is a translucent red or orange-red variety of chalcedony, pale to deep in shade, containing iron impurities.
- Concentric agate: agate showing circular banding in a sliced section.
- Eye agate: in this book an eye agate is one showing one or more areas of concentric bands of alternating colours situated at or near the exterior surface of the agate and which are the visible exposure of hemi agate, qv.
- Fortification agate: an agate having angular markings or parallel zigzag bands resembling the plan of a fortification. Hemi agate: hemi agates are small hemispherical masses found on the inside wall of the clear chalcedony layer. They usually display alternating colour bands, of eye agate.
- Jasp agate: a mixture of jasper (an opaque chalcedony) and chalcedony, in which the jasper ( characteristically red in colour) predominates.
- Lace agate: an agate which has a lacy effect in its patterns.
- Laguna agate: an agate from Laguna Ranch situated at Ojo de Laguna, Chihuahua, Mexico.
- Moss agate: in Scotland, an agate containing inclusions of any colour (but usually green) arranged in moss-like or fern-like patterns. Most moss agate in Scotland is vein agate.
- Onyx agate: a banded agate with straight parallel bands of white and different tones of grey.
- Sagenitic agate: agate containing sprays of acicular crystals (usually zeolites in Scotland) which have been coated by chalcedony. The colour of these sprays is commonly pinkish-red or greyish-white.
- Scenic agate: an agate with markings which form fanciful resemblances to landscapes, seascapes, etc.
- Stalactitic agate: agate in which pendants (generally enclosing stringers of celadonite) have developed within the clear chalcedony layer and which hang down from that part of the clear chalcedony layer occupying the dome or top of the agate.
- Thunder egg: agate from Central Oregon, USA, sections of which often show a pointed or star-like shape.
- Vein agate: agate filling fissures or cracks in various types of rock.
Amygdale.
A gas cavity or vesicle in a volcanic rock, which is filled with secondary minerals, such as calcite, quartz, chalcedony, or a zeolite.
Amygdaloid.
A lava rock containing gas cavities which have been filled with secondary minerals.
Andesite.
A fine-grained extrusive rock composed primarily of plagioclase and one or more of the dark-coloured ferromagnesian minerals (eg, biotite, hornblende, pyroxene).
Basalt.
A general term for dark-coloured ferromagnesian igneous rocks, commonly extrusive but locally intrusive (eg, as dykes), composed chiefly of calcic plagioclase and clinopyroxene.
Cabochon.
An unfaceted cut gemstone of domed or convex form. The top is smoothly polished; the back, or base, is usually flat and often unpolished. In outline it may be any shape.
Chalcedony.
A hydrous cryptocrystalline variety of quartz. It is commonly microscopically fibrous, may be translucent or semitransparent, and has a nearly wax-like lustre. Colour may be white, greyish, pale blue, brown, or black. It has a lower density than ordinary quartz. Some agates have been investigated under the scanning electron microscope. The 'fibres' of chalcedony, visible under the polarizing microscope, are not real fibres (crystal individuals) but consist of su bparticles whose texture is seen to be both granular and platy.
Chemical differentiation.
As used here, the process by which dense homogeneous silica gels develop layers of different chemical and mineralogical composition.
Clear chalcedony layer.
A relatively thin layer of clear or nearl clear chalcedony forming the outermost zone of most agates. It is the first layer in agate to crystallize (by spherulitic crystallization) during its formation. It thus encloses for a time the rest of the uncrystallized agate material (nascent agate).
Colloid.
Any fine-grained material (silica particles, for example) in suspension.
Containment forces in agate.
Many agates are affected by internal dynamic forces which cause distortion of the layers of nascent agate, producing such structures as tubes of escape and dilatations on the tubes of escape. If these forces are strong enough they will also produce rents in the clear chalcedony layer, and the consequent formation of agate dykes.
Crystallization.
The process by which matter becomes crystalline, from a gaseous, fluid, or dispersed state.
Diffusion.
As used here, diffusion is the process of movement of the chemical compounds of silica, water, and pigments through dense silica gels occupying gas cavities in lavas.
Dilatation on the tube of escape.
A balloon-like swelling on the tube of escape (qv) situated near the clear chalcedony layer, to which it generally points. It often contains the same material as the centre of the agate.
Experimental mineralogy.
A branch of mineralogy dealing with the laboratory study of reactions designed to elucidate mineral forming processes. The term includes experiments relating to the physical properties or physical chemistry of minerals, mineral melts, or solutions coexisting with solid or non-solid mineral material.
Geode.
A geode forms when a gas cavity in a lava is first lined with banded chalcedony and then filled incompletely towards its centre with inward projecting crystals which may be of quartz, calcite, or other minerals.
Lapidary.
Lapidary has two meanings: (a) a cutter, grinder, and polisher of coloured stones, such as agates; and (b) the art of cutting gems.
Meteoric water.
Water derived mainly from the earth's atmosphere.
Old Red Sandstone.
A thick sequence of non-marine, predominantly red sedimentary rocks, chiefly sandstones, conglomerates, and shales, representing the Devonian System in parts of Great Britain and elsewhere in NW Europe. The agatebearing lavas of Scotland, some 380 million years old, occur within this sequence.
Quartz.
Quartz, in its narrow definition, is anhydrous crystalline silica, a very common mineral composed of silicon dioxide. It may occur as distinct crystals. It is hard and durable and may on occasion form such gemstones as rock crystal, amethyst, citrine and smoky quartz (cairngorm).
Rent.
The crack or fracture commonly seen in the clear chalcedony layer of agates where this layer is approached by a tube of escape with or without a dilatation. It is a late feature of agate formation.
Secondary mineral.
A mineral formed later than the rock enclosing it, usually at the expense of an earlier-formed primary mineral (a mineral formed at the same time as the rock enclosing it), as a result of weathering, metamorphism, or solution.
Segregation.
As used in this book, segregation means that a feature has formed as a result of the rearrangement of constituents within a dense silica gel after the gel's deposition in a gas cavity; pigments homogeneously distributed in a gel, for example, may be separated into coloured bands.
Silica gel.
A silica gel is a siliceous substance in a colloidal state that allows diffusion of its constituents. No two gels in gas cavities are exactly the same in composition or behaviour and the agates formed from these gels are observed never to be exactly alike in structure, composition or colour.
Silicon carbide grits.
These are loose powders of various grain size (grade) used in. the grinding and lapping of gemstones. Silicon carbide is made by baking together clay and coke in an electric furnace. The powders or grits may also be compressed into wheels or bonded to paper or cloth.
Skin of agate.
The greenish ( or brownish) mineral matter found between the exterior of an agate and the walls of its cavity. One or more silicate minerals may make up this skin.
Spherulitic cystallization.
In this context, spherulitic crystallization occurs in the clear chalcedony layer and is characterized by the dense growth of chalcedony fibres within hemispherical masses which themselves are intricately intermixed, although sometimes separated from each other. Crystallization in succeeding layers is characterized by long, parallel to subparallel, fibrous growths of chalcedony.
Tertiary.
The first period of the Cenozoic era (after the Cretaceous of the Mesozoic era and before the Quaternary), thought to have covered the span of time between 65 and 3 to 2 million years ago. The Tertiary agatebearing lavas mentioned in the text were formed about 50 million years ago.
Tube of escape.
Agates sliced open commonly show chalcedonic bands pinched or squeezed into tube-like forms (tubes of escape) directed towards the clear chalcedony layer where they may swell into dilatations. They sometimes connect with a rent in the clear chalcedony layer, which in turn connects with an agate dyke. A tube of escape indicates that chalcedony or quartz in some agates has undergone deformation just before crystallization. This term and that of dilatation was first coined by M. F. Heddie in 1901.
Vesicle.
In this context a cavity of any shape in a lava, formed by the entrapment of a gas bubble during the solidification of the lava.
Weathering.
The destructive process by which rocks, on exposure to atmospheric agents at or near the earth's surface, are changed in colour, texture, composition or hardness.
Zeolite.
A generic name for a large group of white, colourless, red or yellow, hydrous aluminosilicates that are analogous in composition to the feldspars, with sodium, calcium and potassium (rarely barium or strontium) as their chief metals. Zeolites have long been known to occur as wellformed crystals in cavities in basalt.