What is it?
It appears to be some kind of volcanic rock. It is very similar in
texture to the Hawaiian basalt pictured at the top of this
page.
On Earth, there are at least 3 ways that spherical rocks are formed.
Some spherical rocks are concretions,
like nos. 001 & 050.
Others are rocks that started out blocky with sharp edges and have
been eroded down to a spherical shape by thousands of years of bumping
and grinding against other rocks. Such rocks are usually formed near
the shoreline of large bodies of water or by glaciers. The rock depicted
here was probably rounded by such a process (also, nos. 003,
007, 013, 020,
& 060). Finally, human beings make spheres
from rocks and other materials (probably nos. 023,
064, & 087).
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