My brain is an LA Highway.

A couple key notes. Eridians canonically have a lot of hematite and also silicate in them, based on The Bio Lore Document. Large sections of their carapace would likely be formed accordingly.

There is SO MUCH BLUE! Those really bright light blue Pebbles are visually very similar to both amazonite (a kind of feldspar, which is in that silicate type mentioned in the Lore Document, and relatively hard at a Mohs hardness of around 6), and also some types of jasper (also a silicate) with other impurities.

Amazonite comes in a really wide variety of colour and quality, some of which has brown shades in it. The highest grade stuff is a vivid sky blue that can sometimes also have inclusions like smoky quartz. (If you really want to make Adrian blue and fancy, look up Colorado Amazonite with Smoky Quartz and have at it.)

By the way, labradorite is a type of silicate feldspar, so go ham with that for Eridian OCs. It's valid.

The bright red ones are probably either jasper varieties (higher Mohs around 7) or something more directly related to hematite, just based on a glance. Hematite is supposed to be a lot of the base Eridian carapace, and it's clocking in around Mohs 5.5 to 6.5 depending on the type (just for our reference).

That one in the bottom right is REALLY cool because that looks like basalt with quartz veins. We have a lot of that in my area and I have some in my collection. Basalt is a pretty durable silicate made mostly of feldspar, like amazonite. That one also has green fingers though, and to be honest, I have no idea what those are.

That purple-red coloured one in the middle pic, far left, has some neat legs. That's VERY close to amethyst tone if you have a rough rock that isn't translucent or polished. I'm not sure how viable it would be for an Eridian to form a carapace with a lot of larger crystals like that, because of how they form in nature, but it's entirely possible they've taken in something similar into their diet as they form?

The darker blue in the middle pic, far right are also very interesting. Just eyeballing again, some of those match lazurite, which is what makes lapis lazuli blue. It's a silicate! Could also be a sodalite variety, which is a slightly related silicate of Mohs 5.5-6.

Lazurite isn't as hard a mineral though as others mentioned above (Mohs scale 5-5.5), so either that is a surface marking of some kind that is possibly more pervious to damage ...

Or it's actually meant to be something like sapphire. Which, as a type of corundum like ruby, is harder than anything except diamond (with a Mohs of 9). That would be a dang hard Eridian. Sapphire forms in hexagons usually though, at least of that quality, so ... jury may be out on that too and how it relates to the five-fold-symmetry Eridian bodies prefer. Maybe their shells bring in some more variety than their limb structure. Sapphire is aluminum oxide, which doesn't really fit into the silicates either. Perhaps adornments?

I think the blue dude just to the left of the one we were talking about looks a lot like apatite. I love apatite, but it's also a phosphate, not a silicate, so unless it's a coating again of some kind, likely it's not that. Alas, apatite is pretty but likely not Eridian. Higher chance it's a slightly more green hued form of lazurite. (I have afghanites in my collection that vary in exact blue.)

Now, about Rocky. I've seen a few people say this and I agree -- I think the green gems on Rocky are chrysoprase. It's a microcrystalline quartz (meaning, again, it's a silicate) that often forms in a base stone. It really is that gorgeous, creamy green colour, particularly the higher quality specimens. The green comes from nickel inclusions into the microcrystals.

(Small chance it's a very low grade emerald, but given emeralds are a beryl, and chrysoprase is a silicate, I'm betting on the latter.)

As for his one "mother of pearl" marking, mother of pearl tends to come from organic/inorganic fossil shells. I'm inclined to think it's opal, instead. Which is a silicate.

Also! The jury is actually out on rust or copper patina style oxidization on Eridians, because we don't know how much free oxygen is in Erid's atmosphere. If there's not a lot of free oxygen, they may not experience oxidization the same way some minerals do on earth.

So the chances of the lighter green-blue colour coming from oxidized copper, for example, is probably lower than the Eridian just straight up having a lot of feldspar/amazonite in their carapace.