P W Forster (UK)
I have many years of experience collecting and cutting agates. It was my wife who originally had an enthusiasm for these beautiful semi-precious stones and it was because of her enthusiasm that I developed an interest that has now become an obsessive hobby for the both of us. Cabinets in our home evidence the wide range of specimen stones that an amateur collector can discover. Each specimen has identiﬁcation labels and is catalogued to show the date and the region where it was found.
Before starting my first collecting foray, I obtained as much information on the subject as was available. To this end, I found the book ‘Agates’ by H G McPherson most useful. (This book, together with ‘Agate collecting in Britain’ by P R Rodgers, has been extensively used in the writing of this article.) From my research, it became apparent that the Midland Valley of Scotland contained many of the best deposits of agates in Great Britain. With this in mind, we paid the ﬁrst of many visits to the region.
We started searching along the east coast of Ayrshire. This coast abounds with small coves of pebble beaches and large stretches of andersite larvas that stretch out to sea. During the ﬁrst year, we amassed a large amount of what we thought were agates, but closer examination revealed that we had collected some colourful specimens of jasper as well as some lovely quartz pebbles. This ﬁrst attempt had revealed that those agates that we had found mainly consisted of beach worn fragments. However, we were spurred on to revisit the Ayrshire coast but now with the aim of collecting full nodule specimens wherever possible.
Formation of agates
The agates which can be found in the old red sandstone lavas of Scotland are about 380 million years old from the Devonian period, and are the best treasure houses for British agates. Rocks containing agates are called amygdaloids (or amygdaloidal rocks). This term refers to the lavas containing almond-shaped gas bubbles ﬁlled with secondary minerals including calcite, quartz, agate and chlorite.
The name “Old Red Sandstone” refers (in Scotland) to a thick sequence of reddish non-marine, sedimentary rocks, mainly sandstone, conglomerates, shales and the agate-bearing lavas occurring within these sandstones. These old red sandstone volcanic rocks were laid down, not as one great continuous pile of lava ﬂows, but as a number of thin (1 to 10m), lava ﬂows that issued intermittently from scattered volcanic centres in the Midland Valley of Scotland. When volcanism ceased, for a time, the volcanic ﬂows were partly weathered by natural elements, causing them to alter in composition and to break down into soft rock or even soil. When this occurs, agates can literally be scraped out of this friable lava and makes collecting less strenuous.
Agates can be collected from various locations wherever lavas occur. Many classic sites referred to in H G McPherson’s book have now disappeared through agriculture or have been covered by undergrowth. Quarries are a good source for collecting, and it was often simply a matter of asking permission on the day of the visit to be allowed entrance. However, Health and Safety regulations have put an end to this. If one is fortunate enough to gain access, it will be necessary to dress with the appropriate ‘personnel protective equipment’ (or PPE). On the other hand, it is still possible to search in abandoned roadside quarries. However, I have personally found this to be fairly unproductive. Having said this, Ardownie Quarry, north of Dundee, is a location where exquisite agates have been found.
Farmers are usually quite happy to give permission to search their fields. These can be excellent searching grounds for agates and, if one does not find any specimens, there is the considerable consolation of spectacular scenery. However, it is also worth mentioning that field agates tend to be subject to a great deal of stress, with the fields been ploughed twice a year. In addition, heavy frosts can cause considerable fracturing in specimens.
The coastlines, both on the east side and west side of the Midland Valley, can also produce specimens. Here, the beach worn agates lose their green skin due to the actions of tides. Agates from inland sites tend to resemble any type of rock. However, after a few hours searching, one can become very good at identifying what is, and what is not, an agate. The best conditions for searching tend to be on overcast days and, better still, when it is raining. Water tends to enhance the agate colour, which is predominately blue, and heavy rainfall will remove soil from field agates.
Types of agates
One of the fascinations of collecting agates is the fact that no two agates are ever the same. For this reason, the best way to section an agate will depend on its pattern. This is particularly true of ﬂame agates and piped agates (see below).
These agates can occur in any size. As the name implies, the pattern usually consists of a single line running through either a quartz matrix or a base rock. This particular specimen was from the Cheviots in Northumberland, in the northeast of England.
Stalactitic agate is formed when the agate bands have coated stalactites, which already existed in the roof of the rock cavity before the occupation by silica. The above specimen was collected from the Montrose region.
The term “sagenitic” refers to needle type inclusions of other minerals. It occurs rarely in Britain. I am indebted to the late Gilbert Hall who collected this specimen from the Cheviot area and presented it to me.
As mentioned previously, these agates have been sectioned to reveal a piping or tube formation. This is achieved only when the agate is of a stalactitc type. This particular specimen is an enlarged section from the Ayrshire area.
The typical onyx agate can be deﬁned as having straight, parallel bands of chalcedony, which can be black, white or coloured. These tend to be the most numerous agates and are especially common in the ﬁelds of Montrose.
The term “moss agate” is misleading: firstly, because the moss is a green mineral such as chlorite, which has been trapped within the silica; and secondly, because in many instances, the material in which it is trapped is not agate but un-banded grey/ white chalcedony. Nevertheless, it can be used as descriptive term. This specimen is an enlarged section from a slab found north of Ayr.
This is a descriptive term used where the specimen shows orange or red centres surrounded by red, white or yellow bands.
Fortification agates are so called because they are supposed to resemble the view of a castle from above. This term can apply to many different types of agates. This particular specimen is a ﬁeld agate from Ayrshire.
A geode is a nodule with a hollow centre in which the crystals have had room to develop. The crystals tend to be quartz or calcite. It is possible to have a combination of both and the centre can vary in size.
Faulted agates occur when the agate fractures and is re-cemented whilst still in the parent rock. The re-cementing agents are usually calcite or chalcedony.
Hemi or eyed agate falls into two categories. The ﬁrst consists of those agates which, when part of the outer surface has been removed, reveal white concentric bands surrounding a dark centre (see below). The second, and far more common, consist of those agates, which exhibit small eyes on the outer skin. When cut, these agates reveal an outer layer that has small eye-like patterns. This specimen is a Montrose agate that has had the centre section enlarged for the purposes of this article.
These are examples of the type of agates that can be found along the coast and inland in the Midland Valley of Scotland. It is worth mentioning that agates do occur throughout Great Britain and in many parts of the world where volcanic activity has occurred.
References and further reading
‘Agate collecting in Britain’ by P R Rodgers, BT Batsford Publication, (1975), 96 pages (hardback), ISBN: 978-07134305-6-1.
‘Agates’ by H G McPherson, HMSO Publication, (1993), 72 pages, ISBN: 978-01131001-2-5
‘Studies on Agates: Microscopy, Spectroscopy, Growth, High Temperature and Possible Origin‘, by Terry Moxon, Terra Publications, Doncaster (2010), 96 pages (softback), ISBN: 978-0-9528512-1-9