Spherulite

They are often visible in specimens of obsidian, pitchstone, and rhyolite as globules about the size of millet seed or rice grain, with a duller luster than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure.

As often happens with considerably smaller spherulites, these megaspherulites have been released by weathering from an ash flow tuff, in which they originally formed, to create natural stone balls.

The characteristic radiate fibrous structure is usually conspicuous, but the fibers are interrupted by cavities that are often arranged as to give the spherulite a resemblance to a rosebud with folded petals separated by arching interspaces.

They show, however, that in viscous semi-solid glasses near their fusion point crystallization tends to nucleate at certain centers and to spread outwards, producing spherulitic structures.

Many salts and organic substances exhibit the same tendency, yielding beautiful spherulite crystallizations when melted and cooled rapidly on a microscopic slide.

Variolites are a type of radiate fibrous growth, resembling spherulites in many respects, consisting of minute feathery crystals spreading outwards through a fine grained or glassy rock.