What is the purpose of the green gem/rock in the room with voles of black goo?

Certainly the Goo that David had on his Fingers did seem to have Nano Particles swimming in it, these could indeed be some kind of Life Form/Parasite.

The Earlier Drafts we never had the Goo but Nano-Scarabs, and so its Possible the Scarabs are replaced with Nano-Worm like Parasites.

Above is Sperm under a Microscope we see Many Many what are in effect Nano Cells. So the Pathogen could be similar.

I would NOT suspect the Worms in Prometheus that got Infected with the Goo are related to the Pathogen, i would say they was either some Native Worm/Grub like Organism or one that had been brought by Accident to LV-223 by the Engineers in the Past.

Some Props of what they Intended from Holloways Infection and so we can see the Eye-Worm was not the ONLY such Parasite, their would have been Many, and then Holloway would have started to go through his Transformation.

So looking at those Concepts, and Holloway Props then maybe they was running with the same idea, which could be NOT far from the Original Scarab Idea just change Nano-Scarabs to Nano-Worm like Parasite.

However they are NOT quite Worms..  they are more like the Cnidaria Life-Form Hydra's or indeed Ancient Cystoidea (Primitive Echinoderms).

Predators had green blood, mistery

Michart, but that is something new at least.

I think the most simple explanation is as follows. The green glass like ampoules within the large black organic vessels are essentially organic weapons. The green gem is also holding such a weapon but of more magnitude. Which is what is depicted in the relief emboss directly above the gem (Xenomorph). As the ultimate weapon and the biggest green vessel it’s clearly the organic based nuke of choice, the gold standard alien pathogen aka a Queen (or the means to create one). That’s my theory anyway. (Ps I know this is years later but still re watching in 2022!)