Polyphant stone

Elvan is a name used in Cornwall and Devon for the native varieties of quartz-porphyry. They are dispersed irregularly in the Upper Devonian series of rocks and some of them make very fine building stones (e.g. Pentewan stone, Polyphant stone and Catacleuse stone). Greenstone is another name for this stone and it is often used for parts of buildings such as doorways so they can be finely carved. Most of the elvan quarries are now disused. Others are quarried in bulk for aggregates commonly used for road-building.

Approximately 400 prehistoric stone axes, known as Group 1 axes and made from greenstone, have been found all over Britain, which from petrological analysis appear to come from west Cornwall. Although the quarry has not been identified, it has been suggested that the Gear, a rock now submerged half a mile from the shore at Penzance, may be the site. A significant amount of trade is indicated as many have been found elsewhere in Britain.

...
Wikipedia