Agate Terminology

One of the main attractions of collecting and cutting agates is that no two agates are ever identical. One of the most exciting times is waiting for the saw to finish the cut and pick up the two agate halves and see what has been revealed. Sometimes the most beautiful agate will be revealed. 

It is however possible to see some similarities in the basic pattern of an agate and assign it to one of a number of varieties. This terminology has been established by collectors keen to describe their finds. Many of these terms are jewellers' terms and not mineralogical ones. Some of them may have been established simply to encourage sales and make the agates sound better to potential buyers. 

Matthew F Heddle in his book "Mineralogy of Scotland" [Volume 1, page 58 - 80] did describe a number of different features of agates and different agate types. I have attempted here to cover his points and I have added a few other more recent terms.

Fortification Agate

These agates are so called because the markings consist of more or less concentric bands, which roughly follow the contours of the stone. They assume an angular appearance similar to the aerial view of a castles fortification. Fortification agates are very common and agates showing this pattern can be found in most of the localities in Scotland.

 

Flame Agate


The markings on these stones are suggestive of the appearance of a candle flame. These agates will be a particularly attractive if the colour is red or yellow similar to an actual flame. Whether a flame or other pattern is seen is greatly dependent on the angle the stone is cut.

 

Onyx agate


 These agates show straight line banding through at least part of the structure.The banding is usually parallel to a flattish surface and its orientation usually forms at right angles to the gravitational direction at the time of formation. This fact can be useful in assessing orientation of nearby earth movements. Onyx tends to refer to black and white banding and sardonyx to where there is some hint of colour, usually red or pink. These are often referred to as “water level” agates in America and other parts of the world.

 

Geode Agate


In these agates the whole nodule has not been filled and a hollow has been left at the centre in which crystal have had room to grow to their full dimension. Presumably the mineralising fluids have become exhausted before the cavity filled. These crystals are usually clear Quartz but may be Amethyst or even Smoky Quartz. The nodule may be almost entirely hollow with a thin skin or it may mainly be agate with only a small central empty area where the crystals have grown.

 

Vein Agate


In some areas, notably at Burn Anne, Galston, Ayrshire this type of agate is common. Instead of forming single discrete nodules from gas cavities, the agatising solutions fill fault plains, cracks and other weaknesses in the host rock.

 

 

Moss Agate


In these agates minerals such as Chlorite, Celadonite or Pyrolusite have been trapped within the silica. This silica may take the form of unbanded grey/clear Chalcedony. The term moss agate is restricted to those with green markings. In Scotland occasionally moss agate is found around the outside of some banded agates as from Ardownie Quarry but true moss agate has been found at Scurr Hill near Balmerino in Fife. 

 

Landscape agate


These agates are those which, when cut and polished, reveal by chance patterns that resemble country views. It is said that these types of agates are not common in Scotland but some nice landscape agates are occasionally found. Some of the agates from the fabled “Blue Hole” have shown beautiful landscape patterns as have others from Barras Quarry and Montrose. Even if the pattern within an agate is not suggestive of a landscape sometimes beautiful coloured abstract patterns can be found. 

 

 

 

Eye Agate


It is presumed that this phenomenon occurs when small concentric bands of agate form around small projections on the inside wall of the original rock cavity. These bands may be visible on the outside of water worn agate or if the outer shell is removed. If the agate is then cut though this eye, a half sphere is visible on the perimeter of the agate’s cut surface.  

 

 

Stalactitic Agate


The occurrence of a stalactitic pattern within an agate is essentially dependant on the angle of the cut. These stalactite-like bodies have the appearance of descending from the upper edge of the cut face of the agate. They form when agate material coats small stalactites, which already exist on the roof of the rock cavity before the whole cavity is occupied by silica. In some agates the original material which formed the stalactites can be seen. In others however there is no trace of this original material.  

 

 

Piped Agate (Tube Agates) 


Occasionally the stalactitic agates are cut across the stalactites and the resulting pattern shows the cross section of the stalactite. Good examples of these can be seen in the agates from Burn Anne in Ayrshire.

 

 

Faulted Agate


Faulted agates are agates that have been disrupted in some way while still held within the original rock and then recemented. What the mechanism of this disruption is is not clearly understood but may represent settlement of rock deep underground or even nearby earthquakes or tremors. The re-cementing agent is usually Calcite or Chalcedony.  Usually these agates have been faulted and re-cemented in more or less the same position but occasionally the faulting results in dislocation of the cavity and the agate may then be re-cemented in a different position. 

 

Brecciated Agate


This is the extreme of a faulted agate where the agate has been effectively shattered by whatever force that was brought to bear and then re-cemented in a random totally different position.

 

 

 

 

Mocha Agate


Mocha agate is like Moss agate only the inclusions give the impression of a brown colour rather than green. These small brown inclusions may be small particles of manganese dioxide. Mocha vein agate has found at Maidens on the Ayrshire coast.  

 

 

Sagenitic Agate


Sagenitic agate describes agate that contains needle-like inclusions of other minerals. Occasionally this mineral may be Natrolite.

 

 

Jasper "Agate"

This term is used to describe agate that is composed of opaque material and not clear bands of chalcedony. In Scotland such material has be found in the Campsie Hill north of Glasgow and even in the Carboniferous lavas of the Birrenswark series. Stricktly speaking this is not a correct term and Jasper is certainly not Agate.

 

 

Dendritic Agates

 

 

 

 

Disc bearing and Ovoid bearing Agates

 

 

 

Iris Agates

 

 

 

Plume Agates

 

 

 

Tube of Escape

 

 

 

Dilatation on the Tube of Escape