Some articles on megaspherulites:

Spherulite - Megaspherulites
... are more than 20 centimeters in diameter, are called megaspherulites ... Near Silver Cliff, Colorado, megaspherulites, which range in diameter from 0.30 to 4.3 meters occur within a thick layer of rhyolitic vitrophyre ... Megaspherulites as large as 0.91 meter occur within rhyolite exposures on Steens Mountain, Oregon and ones as large as 1.83 meters in diameter occur within welded tuffs exposed ...