Some articles on lithophysae:

Spherulite - Lithophysae
... Very large and cavernous spherulites are called lithophysae they are found in obsidians at Lipari, in Yellowstone Park and other places ... Some of these lithophysae are several centimeters or more in diameter ... Tridymite, fayalite and other minerals in the lithophysae may be precipitates from the vapor phase that occupied the cavities ...