Another strange 'cup' on Mars

A Curiosity close-up view of another 'cup' feature like those in my last blog post (contrast stretched to show color variations):

Lava bubble on Mars? Concretion with a soft center?

(original Mastcam image,  0122MR0765000000E2_DXXX.jpg)

The shallow slopes on some sides, and hollowness make me think that they are more lik bubbles, trapped while rising through molten rock. Are these flat outcrops consistent with lava flows? How come they aren't common on earth?

Perhaps the apparent spheroid, below another example of a 'cup', is one of these 'bubbles' that is intact:

JPL and Curiosity request: Please laser blast and and then attempt to pop this bubble with your arm.

Update, 12/18/12:

Chem cam is on the case. Here's a telescopic closeup of the shell-like top edge:

Dammit Jim, I'm a geologist not a doctor! (The Devil in the Dark) These could be Horta egg shells.

Set lasers to stun.