Rock of the Month
Paradise Valley, Morgan Hill
Santa Clara County, California, USA
“Poppy jasper” is a trade name for a particular variety of orbicular jasper that is characterized by large, variously conjoined spheroidal and ellipsoidal orbicules. Orbicular jasper forms when rhyolitic ash cools exceptionally quickly. Just as big equant grains of quartz and alkali feldspar crystallize from slow-cooling granite magma, so radial aggregates of needle-like crystals of quartz and alkali feldspar under rare conditions crystalilize from a fast-cooling rhyolite ash.
In the specimen above from Morgan Hill, at the southern end of Silicon Valley, wedged between the San Andreas and Calaveras faults, each orbicule is banded concentrically with brick-reds and yellows and enclosed in a yellow corona.
After the “poppy jasper” was exhausted in the mid-twentieth century, a fool dynamited the seam, shattering it into fragments too small for most lapidary work. As a result, the Morgan Hill variety of “poppy jasper” is represented in today’s market only by recycled specimens from old collections.
Updated April 17, 2015