9500 year old obsidian bracelet shows exceptional craft skills

Past Horizons ^ | Tuesday, December 27, 2011 | LTDS press release

[aruanan]

Corundum (also known as carborundum, or emery) is a grinding and polishing material and although very hard (no. 9 on the Mohs hardness scale in which no. 10 is the highest value), it is easy to work in mineral form.

I think rubies and sapphires are also corundum. I have, in some box somewhere, what looks to be a very weathered bipyramidal corundum crystal about the size of a tennis ball I found in a load of gravel in the Dominican Republic (that is, I hope it's there and that someone didn't swipe it from my desk in the lab). Carbide won't scratch it.

[Bernard Marx]

I can’t even imagine how it was shaped. Obsidian shears sharply

It grinds pretty easily because it's relatively soft although it fractures readily under pressure or a hard knock. I've cut and polished lots of it, especially iridescent "rainbow" obsidian.

I wish the image of the bracelet was better quality. I can't make out if it's a solid piece or made of tiny tiles. I suspect some sort of primitive lathe was used in its grinding and polishing in order to achieve the precise surface measurements and finish. The concept of the lathe is simple and very ancient. Potters' wheels are a form of lathe.

[mylife]

The fracture surface of obsidian is many times sharper than the finest surgical steel

Indeed. LoL

I just had to be an ass and use a rock to fix that system, just to make a point LOL

I'm kind of a jerk that way! LOL

[mylife]

Page 164 Foot powered lathe

[mylife]

Page 164 Foot powered lathe

[Bernard Marx]

I think rubies and sapphires are also corundum.

They're the gem-quality forms of corundum but most of corundum (aluminum oxide) is sandpaper-quality.

Is your crystal hexagonal? What color? The Dominican Republic seems a very unlikely source for crystalline ruby or sapphire, especially of that huge size. When you say carbide won't scratch it, what form of carbide have you tried?

[aruanan]

Is your crystal hexagonal? What color? The Dominican Republic seems a very unlikely source for crystalline ruby or sapphire, especially of that huge size. When you say carbide won't scratch it, what form of carbide have you tried?

If this is corundum, it's the bipyramidal form, not the hexagonal, flat on either end form such as I've found in North Carolina. I've also seen some other bipyramidal crystals of corundum, but each side looks to be 2x height to base. The sides of mine are more equilateral. There's definitely corundum in the DR, just not anything very large that I've heard of. The carbide I used is tungsten carbide. I use a diamond wheel to cut it (rotary files). I used the same diamond wheel to grind a smooth place on the fractured end of the stone. A carbide point drawn across this surface leaves a mark, but it looks like a faint line of carbide left on the surface and not a scratch in the surface. I used a little piece chipped off the bottom to scratch glass and a piece of quartzite. It's sort of a taffy color and glossy in appearance. I don't know what it is, but when they dumped a load of river gravel on the playground I went to look through it. There's a lot of cool stuff in the DR. This stone immediately caught my eye.

[Bernard Marx]

Corundum crystals can be bipyramidal but they're *always* in the hexagonal subsystem of the trigonal crystal system. If they're not the mineral isn't corundum. Count the outline of crystal faces in line with the camera lens in the bipyramidal example you attached. Six, right?

Crystal "habits" can vary a lot; here are examples: Corundum Crystals

It's certainly possible for corundum to be found in the DR. Regions near subduction zones can offer a cornucopia of interesting minerals. I said it's an "unlikely source," especially for huge crystals like yours, because I'm pretty avid on the subject and the DR isn't known for corundum. It sounds as if your hardness-testing methodology is pretty accurate. Do you have another piece of known corundum around you can use to try another scratch test? Or a piece of topaz? Topaz is harder than quartz and chrysoberyl is harder than topaz.

The specimen sounds fascinating. Can you take it to a mineralogist for a positive ID?

[aruanan]

It’s bipyramidal and octahedral. About 2/3 of the other end is missing. I was going to take it over to the geology department here but didn’t get around to it. Now I can’t find it. I hope I took it home before my move and it’s in some box somewhere.

[Bernard Marx]

If it's truly octahedral it's in a restricted class of minerals. The common translucent-to-transparent ones that crystallize as bipyramidal octahedrons are diamond, garnet, fluorite and spinel. They're all in the isometric (cubic) system.

Likely possibilities from that group are garnet and fluorite, but both are much softer than your hardness tests indicate. If you could grind it easily with a diamond wheel it isn't diamond! Spinel is unlikely but possible. That would be an very unusual spinel color. I hope you find it and get an ID. I'd be interested in knowing.

[aruanan]

If you could grind it easily with a diamond wheel it isn't diamond!

It could be ground, but not easily, though it didn't blunt the cutting of the diamond wheel like an actual diamond does. We had an opaque greenish diamond of about the size of a big pea that we used to slow the cutting of the side of the diamond wheel after dressing it. And grinding with a sand wheel or those black composite grinding wheels didn't cut it. Do any of these minerals develop weathering that is not as hard? As to the shape, if you look from the top, it is square and the top four sides are, as I said, pretty much equilateral. If it had sharp corners and vertices, they're rounded now to about a half inch or so, except the bottom pyramid, which is broken off.

On another note, when I was there in the Cibao Valley, I knew some people who knew some people who took us up to an amber mine in the mountains north of Santiago. The strata were dark grey between much lighter grey and about 4 or 5 feet thick. The mine shaft was dug into the darker grey. You had to goose walk to go into it. The stratum had widely spaced thin dark lines that got closer and closer together until at about the middle they met to form a thicker, more irregular band. Within that band were nodules of amber. I got a couple of pieces with what look like termite wings and some little ones with a tiny serrated, sort of heart-shaped leaf and stem and what looks like the tip of an evergreen branch. I bought one piece for my brother to sell to a friend of his. It was about 3" X 3" X 1.5" and still had some of the matrix stuck in places. I think it cost about 100 pesos.

I hope in the future to visit a friend of mine that lives in the state of Goias in Brazil. His city is right near the border of Minas Gerais. He said that near his home are two old diamond mines. That region looks really, really exciting from a mineral and gem point of view.

[mosesdapoet]

Am aware of the wandering Antartica you mentioned before it wound up where it is presently at which was after “the great split” . I believe that was prior to the 2nd last ice over, around 40-50,000 years ago .

[<1/1,000,000th%]

Line c looks like the Phoenician alphabet, although scrambled out of order.

Greek was derived from it.

[Beowulf9]

Gorgeous!

When you meet a mug lately out of the jug
And he’s still lifting platinum folderol
Call it hell, call it heaven
But it’s probable twelve to seven
That the guy’s only doing it for some doll.

[SunkenCiv]

Just an update.

Using a technique for analyzing friction in industrial equipment, a group of French and Turkish scientists have unraveled the process that was used approximately 10,000 years ago to make a highly polished obsidian bracelet. The team examined a bracelet fragment from Asikli Höyük in Turkey at different levels of magnification and saw evidence of three stages of production -- pecking, grinding, and polishing. Striations on the bracelet indicate that a mechanical device may have been used to achieve its regularized shape and glossy finish. It is the earliest evidence of such a sophisticated stone-working technique. [ The Neolithic Grind by Zach Zorich, Volume 65 Number 3, May/June 2012 ]



(Courtesy Obsidian Use Project)