Napoleonite

The grey matrix of the stone has the normal appearance of a diorite, but contains many rounded lumps 1 or 2 inches in diameter, which show concentric zones of light and dark colors.

[1] Spheroidal structure is found in other diorites and in quite a number of granites in various places, such as Sweden, Russia, America, Sardinia and Ireland.

The nucleus for the spheroidal growth is sometimes an early porphyritic crystal, sometimes an enclosure of gneiss, et cetera, and often does not differ essentially in composition from the surrounding rock.

Some are merely rounded balls consisting of the earliest minerals of the rock, such as apatite, zircon, biotite and hornblende, and possessing no regular arrangement.

As a general rule the spheroids are more basic and richer in the iron-magnesium minerals than the surrounding rock, though some of the zones are often very rich in quartz and feldspar.