The Ffestiniog region contains five major slate veins, which are on top of a thick layer of feldspathic porphyry, called the "Glanypwll Trap".
The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.
The name is taken from that of the Amazon River, from which certain green stones were formerly obtained, but it is doubtful whether green feldspar occurs in the Amazon area.
It was first described from complex crystals and as broken fragments in the disintegrated material of a granitic vein at Stoneham, Oxford County, Maine where it is associated with feldspar, smoky quartz, beryl and columbite.
Donnybrook stone is a fine to medium-grained feldspathic and kaolinitic sandstone found near the town of Donnybrook, Western Australia.
In later years he devoted himself largely to petrology, and published memoirs on the trachytes of Hungary and Transylvania; on a new method of determining the species of feldspars in rocks, depending on fusibility and flame-coloration; on the geology and petrology of the district of Schemnitz; and on Santorin Island.
Yet Lorence G. Collins does not agree with the assumption of the K-feldspar being primary magmatic and the myrmekite being formed due to deformation-induced Na-Ca-metasomatism.
Another variety more frequently used as a gemstone is the aventurine-feldspar or sun-stone found as reddish cleavage masses in gneiss at Tvedestrand in southern Norway; this presents a brilliant red to golden metallic glitter, due to the presence of numerous small scales of hematite oriented within the feldspar structure.
Slip is an aqueous suspension of a clay body, which is a mixture of clays and other minerals such as quartz, feldspar and mica.