“...These extraordinary and predominantly Jasper-Agate type nodules and geodes are formed within the Triassic marls and conglomerates of the Mercia Mudstone series, lying uncomformably over the Carbonifierous limestone’s of the English West Country and South Wales. The geodes which range from the size of a pea to something the size of a medicine ball or even bigger are formed as silica replacements after vugs created by the formation of salt or gypsum originally as nodules. The Geodes themselves are invariably lined with quartz crystals of various hues (amethyst, citrine, smokey quartz etc) and often have mineral infills of barite, calcite, Celestine, Strontianite or other species. The agate owes its colouration to the Iron oxide and other metallic oxides which are abundant in the red marl. The stones when found are more usually referred to as "Potato stones" as they commonly resemble Potato’s when recovered especially from cultivated fields.
The banding is commonly concentrical (having eyes) but can also be wavy with lacy tight swirls or even bear parallel bands. The colour of the bands are commonly orangey, reddish, whitish or pink but can also be yellow, purple, grey, brown or pale blue. The potato stones are commonly found in cultivated fields in parts of South Wales (Glamorgan, Gwent) and around the Mendip hills of Somerset and Avon. They can also be found as far north as Gloucestershire and as far West as Pembrokeshire in South West Wales. The occurrence of Potato stones is not just exclusively to fields either. They can be dug for out of the ground and have been found in several quarries and cuttings in the past as well as some of the beaches along the South Wales coast...”
personal report of Paul Render, GB
More informations about english agates are given in Dr.H.G. Macpherson´s book “agates” (published by the British Museum (Natural History) and the National Museums of Scotland) at page 52.