FMF - Friends of Minerals Forum, discussion and message board

[SteveB]

I’m thinking much along similar lines as others, quartz based, olivine related. My understanding with measuring refractive index is usually requiring a pure crystal, ie. good clarity as well as a polished flat surface to place against the device. Inclusions internally will throw off the reading. I'm more knowledgeable with optics where the light beam enters the front of the sample directly with no air gap and travels into the medium and internally reflected back with deflection angle measured. Light “bends” refracts at every transition between mediums. So here you have the specimen, air and a special oil (made to ensure no air gap with little deflection. Its all geometry and depending on the method used it can be read from the front in relation to the light source or behind where a linear light source is expected to reach a detector. Its easier to linear polarize the light and use spectroscopy method to measure reflected deflection.

Ouch, my brain hurts. Anyway what you thought were crystal facets I don't think are related to crystallography, but are more related to the geometric fracturing of rapidly cooling materials. Think mud cracks or basaltic columns. So I would be looking at minerals often associated with volcanic magma flows into water where it cools more rapidly and cracks at the surface of the lava.

[Pete Modreski]

Hi, M,

I've read all the responses to your question, and people's ideas seem to be circulating around a similar group of minerals. When I looked at your photos and read what you said, my first thoughts were, epidote, and olvine.

If your high (>1.7) refractive index measurement was at all correct, it would suggest epidote. But you do really need to have an optically flat, polished surface (like the table facet of a gemstone) to get a reliable refractive index reading. [One reads, " Epidote has a high refractive index, similar to pyrope garnet."

My comment, your photos, including the "sliver", seem overall too dark green to likely be just green-included quartz. Just my impression! But, photos can be deceptive, in trying to recognize a mineral.

Good luck with further progress in confirming what it is!

P.S., just an amusing sideline, about how "one must be careful about what one reads", when I typed in to Google, "refractive index of olivine", the first thing that came up was a picture of a faceted peridot with, in big bold lettering, "Peridot / refractive index 2.63-2.65". That iss totally wrong--those numbers are the specific gravity of peridot, not the refractive index!

[SteveB]

Thinking further I'd be looking into the Serpentinite group for the answer.

[MKomishyn]

I borrowed a presidium gem tester 2, and it reads as jadeite. That sort of fits with all of the other properties, and jadeite is found in NC (not often). I will post back after I get my triple-beam scale so I can measure specific gravity. Though, I'm leaning heavily toward jadeite after reading all of its properties (vitreous to greasy, around 6.5 mohs, splintery fracture, and I think it shows cleavage, though those faces could have been because they were crammed between two other crystals, I suppose).

[alfredo]

Those gem testers are designed for use on polished gems. They don‘t really work well on rough stones.
But there is another test you can easily do yourself - Fusibility. See description in a Mindat article: https://www.mindat.org/article.php/883/Jade

[Pete Modreski]

I wanted to add this too, as per what Alfredo said. In addition to needing a smooth (preferably polished) surface (I don't know how they'd perform on a rougher surface), there are only a dozen or so minerals shown on that meter's scale, and of course there are "hundreds" of possible minerals/gemstones. As you say, M, jadeite is not common in NC, and I think it would be very rare to "scarcely ever" to find a piece of jadeite in a stream bed there. I think specific gravity will be your best bet, to at least eliminate some mineral possibilities, and give you a list of "possibles".

[MKomishyn]

Thanks, everyone. Specific gravity was 2.62 (crappy scale but confidently it is not anything else besides green quartz / aventurine). Thanks so much!

Solved: Green quartz or aventurine