A quarry which has produced many extraordinary “potatoe agates” is located near the village of Dulcote.J. Zenz writes in his book "Agate", bode-publishinghouse 2005:
“...A very interesting and highly productive finding-location in the past where of many agates and agate geodes could be found is located in the area of the village of Dulcote, about 2.5 km east of the city of wells in Somerset in southern England, known by its wonderful cathedral from the 12th century. A large quarry which is pursued here in a part of the Mendip hills, is producing in the year up to 240,000 tons (!) of Limestone from the Triassic period for construction and road-gravel purposes. On the other hand, the quarry is again filled with garbage. ...
In the southern area of the quarry especially in the 1970s and 1980s mighty red layers of clay were uncovered, overlying the limestone These layers included a lot of nodules. While similar scattered finds from further outcrops in the Mendip hills and fields were known, the agate on this layer of quarry in particularly concentrated and rich number occurred. This wealth of material moved in the late 1970s the Foster Yeoman Company Ltd. to start a commercial mining and processing. The interesting balls were sorted, cutt and polished in small, factory in Winchester. Apart from collectibles and slabs, especially balls, eggs and cabochons made were made from the material (...).
The size of the potatoes was between 3 and 45 cm, specimenwith a diameter of 20 cm occur relatively frequently. ... Many of the nodules are hollow and host some very nice formed calcite, rock crystal (sometimes as doubly terminated crystals), smoky quartz, amethyst, goethite, galena and celestite. Geodes filled completely with calcite or a non-descript corroded material are not uncommo. ... The predominant colors are a strong purple, blue, orange, and rarity also violet. Strong color contrasts and bizarre drawings with “eyes”would be particularly to mention..."
More informations about the formation and the occurence of this nodules can be found at the page Mendip Hills.