[Aracelis]

"concretions" . . . as in requiring a liquid environment to form? Past liquid environment is where my thoughts track.

Yes, it would require an aqueous environment according to the geology that we are familiar with (Earth). Here's a good link to a page describing concretions:

Concretions, Thunder Eggs and Geodes

[Phil V.]

thanx . . . tomorrow's read . . . gnite!

[Don Joe]

Concretions, Thunder Eggs and Geodes

The two geodes in the upper-right of that page (repeated at the bottom) are reminiscent of the Mars spheres, they even have the hole at the end, but they lack the "peach line" that so many of them feature. Can you think of any explanation for that "peach line" -- something that would explain why it's on so many of them? (Possibly all, since we see at most only half of each sphere.)