Ask A Geologist - Miller Museum of Geology
Generated: 14 June, 2017, 14:36
Black Jasper
Posted by Laura Paul - 2015/02/25 16:55
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Hi, I'm a beginning lapidary artist and I recently bought a pendant that was made with locally found black
jasper. The vendor explained that the black jasper could be found on any of the beaches near here
(Gaspe/Perce, Quebec, Canada) however with the language barrier he was unable to explain to me how
I could distinguish black jaspers on the beach from other black rocks. I was hoping that someone would
be able to tell me some characteristics that I should be looking for. I have found a large black rock that
seems to feel heavier (it might possibly only be in my head) than other rocks and it 'glitters" (for lack of a
better word) only a tiny bit and only by spots. I have tried to capture this with the camera, but I'm not
certain I've done a great job if anyone is willing to take a look and provide a 'diagnosis' that would be
awesome. Either way, any help I could get in trying to distinguish black jasper from other black rocks
would be greatly appreciated. Thanks so much in advance for your help and have a great day,
Laura
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Re:Black Jasper
Posted by mbadham - 2015/03/02 11:33
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Hi Laura
I'm afraid there isn't going to be an easy answer to your question, and it has to do with the different ways
that the lapidary world and the scientific world classify rocks and minerals.
To the geoscientist, "jasper" implies that it is a very fine grained ("microcrystalline") material composed
mostly of silica (chemical formula: SiO2). These microcrystalline materials can often have impurities in
them (other minerals, sediments, elements), which means that they can be many, many different
colours. To a geologist the name jasper actually implies that it is a brown, yellow or red colour of
microcrystalline quartz only. A black microcrystalline quartz would most often simply be termed a
"chalcedony" (cal-sed-o-knee) by geologists. One important distinction for jasper is that it is opaque and
not transparent or translucent like some other varieties of microcrystalline quartz.
Microcrystalline quartzes that have patterns in them are termed "agates". Chalcedony, agate, jasper,
flint, chert etc. are all called "varieties" of microcrystalline quartz in geology, and the distinctions between
them are fairly arbitrary based on colouring and patterning. They are all mostly made of SiO2 though,
with impurities and different formation environments that cause the different appearances.
The lapidary world doesn't really look at a rock or mineral in terms of its chemistry or composition
though. They mostly look for the colours/patterns/textures, and the physical properties of the sample i.e.
how hard it is, how it breaks, how high a polish it takes, etc. This leads to some confusion in names
because they use familiar geological names like "jasper" and apply them to different materials that have
similar properties but are not necessarily microcrystalline quartz.
I am not totally knowledgeable about the different localities for "black jasper", but it seems that Brazil
actually produces a black microcrystalline quartz material that is marketed as black jasper. A quick
review of the Internet shows that many other hard black rocks of unknown composition are maybe also
being sold as "black jasper" however.
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Ask A Geologist - Miller Museum of Geology
Generated: 14 June, 2017, 14:36
The material that your pendant was made from could actually be a black chalcedony found in the Gaspe
area. Nodules of microcrystalline quartz of various colours are fairly common in many rocks. In order to
be sure of its identity though, someone would have to do a chemical analysis to see if it is mostly silica
(SiO2).
For your purposes, you can probably rely on the following physical properties for identification though:
1/ it is a black colour on a freshly broken surface (the weathered outer surface of a nodule is often
lighter-coloured.)
2/ the material breaks with a conchoidal fracture (flakes off leaving scalloped shapes like glass does
when it chips on an edge: e.g. en.wikipedia.org/wiki/Conchoidal_fracture
3/ it is very hard. Get a piece of scrap window glass, place it on a flat surface like a table(don't hold it in
your hand) and see if the mineral will easily scratch a line on the glass. It should easily leave a scratch in
the glass, and not just the powder from a softer rock breaking up against the harder glass.
4/ it shouldn't have any visible minerals or crystals in it since by definition jasper is "microcrystalline" and
should just be uniformly fine-grained. This will rule out most black rocks.
5/ it shouldn't be unusually heavy since the density should be roughly the same as quartz at around 2.6
grams/cm3.
As usual with my answers, I have given a pretty long-winded explanation here, but I hope it helps. Here
is a web page that has some phrases that make the scientist in me cringe a bit, but overall it captures
the lapidarists view of the different varieties called "jasper":
www.gemselect.com/gem-info/jasper/jasper-info.php
Mark
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