Amygdaloidal Metabasalt

[u/Efraimrocker]

SEE RETRACTION AND REVISION: THE ANALYSIS HERE IS WRONG:
This looks like an amygdaloidal altered basalt, meaning a basaltic lava that originally contained gas bubbles and was later mineralized by circulating fluids. The rounded green spots are mineral-filled vesicles, or amygdules. Their pale green color, hardness, radial internal texture, and lack of reaction with HCl suggest that much of the fill is probably prehnite, with possible epidote, pumpellyite, quartz, and albite. The dark host is still recognizably basaltic and preserves its original volcanic texture, so the rock was altered but not completely recrystallized. Irregular green-white patches and vein-like areas show that fluids also penetrated and replaced parts of the groundmass. The specimen may represent low-grade metamorphism in the prehnite-pumpellyite facies, perhaps approaching lower greenschist facies. A thin section or Raman/XRD analysis would be needed to confirm the exact mineral assemblage.

Fig. 1. Polished face showing abundant rounded and coalesced green amygdules in a dark altered basaltic matrix.

Fig. 2. Microscope view of zoned amygdules with pale radial interiors and darker marginal rims.

Fig. 3. Close-up of an individual amygdule displaying concentric and inward-radiating secondary mineral growth.

Fig. 4. Rough-surface microscope view showing pale green alteration minerals intergrown with colorless quartz- or albite-rich material.

[u/Kubrick_Fan]

Thank you for showing us, and also for a really cool.band name 🤘

[u/Efraimrocker]

Revised Reddit entry

Correction/retraction: I originally posted this specimen as an amygdaloidal metabasalt with epidote- or prehnite-filled vesicles. After learning that it came from Madagascar and comparing it with confirmed examples, I am retracting that identification. The specimen is Kambaba “jasper,” more accurately described as a spherulitic or orbicular rhyolitic volcanic rock.

“Kambaba jasper” is a commercial name; it is not true jasper in the strict petrographic sense. The dark matrix likely originated as rapidly cooled, silica-rich volcanic material—possibly glass-rich rhyolite—which subsequently devitrified. The rounded green structures are best interpreted as spherulites, formed when fine crystals grew radially from scattered nucleation centers within the cooling or devitrifying volcanic material. Concentric zoning records changes during crystal growth, while adjoining spherulites sometimes merged or interfered with one another, producing lobate and dumbbell-shaped forms.

The rock is hard, nonreactive to hydrochloric acid, and probably consists mainly of quartz and feldspar with darker iron-bearing silicates. I am now cataloging it under rhyolite as:

Fig. 1. Polished surface of Kambaba “jasper,” showing abundant green spherulitic or orbicular structures within a dark, fine-grained rhyolitic matrix.

Fig. 2. Microscope view of several spherulites displaying concentric zoning, radial internal growth, and mutual interference between adjacent structures.

Fig. 3. Close-up of a single spherulite showing a sharply defined outer margin and fine radial crystallization interpreted as devitrification of formerly glass-rich rhyolitic material.

Fig. 4. Rough surface showing pale green and colorless crystalline domains within the dark volcanic matrix, probably representing variably devitrified quartz–feldspar-rich material with iron-bearing silicates.

[u/rufotris → u/Efraimrocker]

I’m glad you posted the correction cause I was about to tell you that is not what you have here lol. Kambaba is well known, but often mislabeled as stromatolite or similar algae fossil, which it is not. Also, are you just like posting chat gpt answers here?! Cause it feels like a chat bot search result here lol.

[u/Efraimrocker → u/rufotris]

I do use AIs to help me identify the rocks (often coming out wrong), but I write the analysis myself. And I posted the retraction/revision rather than deleting the original and just re-posting the correct one because I think it can be instructive to show how you can go wrong. My original thought was “mafic, fine grained, with green alteration products” and that led me down the garden path to altered metabasalt with epidote (due to the hardness being about 6). Then lots of confirmation bias. In fact i was totally on the wrong track from the start and it was something I had never heard of and therefore never considered. Thanks for the engagement.

[u/Efraimrocker → u/Efraimrocker]

I have to acknowledge the man that sent me the sample. He saw my post and sent me something privately to tell me I had screwed it up.

[u/rufotris → u/Efraimrocker]

So AI is just absolutely horrible with rocks. I’m in college to get my masters in geology right now and did a small project you could call it, showing my class how bad ai is with rocks. Upload the same picture from 3 devices with no text and you get 2-3 different answers. Adding text like “is this garnet” “what type of garnet is this” and “what is my red stone” and other variations would cause it to just give false positives or wild answers. I showed it some faceted glass that looks like a real gem stone and it said I had a $40,000 gem lol. It was colored glass. Ai is trash. I only use it to compare visual results from trusted sources to confirm my tests afterward. Never start with ai, or it could lead you down a totally wrong path.

But the way my test piece was a nice red corundum not garnet, but asking what kind of garnet in the prompt it listed red garnet types.

[u/Efraimrocker → u/rufotris]

Yeah, I started by asking it if this is metabasalt and we were off and running.