Thread: Question for BBW/Coticule Gurus

Originally Posted by JNT

I recently won this combo BBW/Coticule off ebay. To me, at least, it is a beautiful hone. Beautiful patterns and a very cool coming together of the BBW/Coticule, can be seen on the sides. I have a few combo hones, but I have never seen the BBW side (on other combo hones) look like it does on this one. There are rather good sized patches of deep/dark blue in the surrounding brown of the BBW. I am including a couple pics to show you what I mean. Can anyone tell me what these patches are? And are they a bad thing in a BBW?

Thanks!

Your pic #3 shows a highly irregular boundary suggesting that the stone has to be natural (or I'll eat my hat). It looks like the part of the mine where this thing came from had some slippage or faulting and/or was more highly metamorphosed than the other areas, resulting in this folding of the two strata. The round patches - and possibly the darker areas at the contact - may be areas of concentrated garnets. You can check with a handlens to see if you can find cubic crystals, or 12-sided critters in there. Nice looking stone....

JNT, I have no knowledge value to add, but your third photo showing the beautiful, irregular line between the yellow and blue is just killing me! That is a stone of great character, and rare. You will look a very long time before you find another like it.

I love coticule naturals because at bottom, I'm a simple man. I just don't understand how such an abrupt, yet flowing and arbitrary stone and color boundary could have been formed many millions of years ago. And that somebody found it, appreciated it, dug it out of the ground at substantial personal safety risk, and used it to improve their life in a very universal but very specific way.

The thing that kills me is how such a pretty, purple, figured stone can be useful? There are pretty purple stones (gems, etc.), but how many of them are pretty *and* useful? I love 'em, yes.

Originally Posted by Oldengaerde

The Coticule side is mostly hard and inclusions of quartz and/or wavy quartz(like) lines parallel to the joint line are not uncommon.

A little knowledge being a dangerous thing, I knew that the abrasive in coticules is garnet but I didn't think coticules contained quartz. So when Artan posted the above I googled 'composition of coticules' and came up with this here .... quoted below ;

The Lower Ordovician in the Caledonian Stavelot Massif (Belgium) contains a coticule member, probably Arenig in age, composed of red hematite-rich shales alternating with thin yellowish coticule beds. These coticules are fine-grained highly manganiferous metamorphic rocks mainly composed of spessartine garnet, mica and quartz. Similar rocks of about the same age occur in several localities along the Appalachian and Caledonian fold belts. The Belgian coticules are thought to have originated from marly sediments which had been deposited rapidly, probably by density currents, and in which CaCO3 had been diagenetically replaced by MnCO3. During metamorphism, spessartine garnets were formed from this MnCO3 and mica. Manganese and iron are thought to have originated from volcanic-exhalative processes.

^ Nice reference. I tell you though, their description seems to be mostly about the wall/host rock (BBW) and less about the coticle lense. The two have similarity, but are so different in many ways. I think the origin of the coticle itself is still controversial.

In a couple of weeks, I will measure the mineral phase composition of my natural. In addition to the spessartine and quartzite contents, we should be able to see what comprises the matrix of the stone (mica and/or other clays, any carbonate species or hematite, crystallinity etc.). I should have a bout of this silkvein (SA) hone to check out as well.

There is one other 'coticle' deposit that I know of - at least a similar high-grade spessartine-quartzite deposit that has been called coticle - and that's in NE Nova Scotia. Look familiar?...

Originally Posted by Woodash

In a couple of weeks, I will measure the mineral phase composition of my natural. So, in addition to the spessartine and quartzite contents, we should be able to see what comprises the matrix of the stone (mica type(s), any carbonate species or hematite, crystallinity etc.). I should have a bout of this silkvein (SA) hone to check out as well.

Wouldn't it be interesting to check out a number of coticules ? Particularly those that are obviously from different strata and mines based on their characteristics judged by color variations looking at them from the side.