Jasper

 

last modified: Wednesday, 15-Feb-2012 01:48:47 CET

Document status: incomplete

Jasper is the name of dense and opaque varieties of microcrystalline quartz. Jasper is not really a mineral in the strict sense, but a mixture of different types of microcrystalline quartz with impurities of other minerals, and is called a textural variety of quartz. Jasper of homogeneous color looks a bit like a colored, opaque flint, and shares many of its physical properties, but it forms in different environments. Multicolored jasper makes for an interesting ornamental stone, and red jasper is cut as a gemstone.

 

Specific Properties

What clearly distinguishes jasper from other valuable cryptocrystalline varieties like agate, chrysoprase, sard, or carnelian, is its opacity. Only thin chips of jasper are translucent. It might, for example, be of the same color as a carnelian, but the latter is translucent. Heliotrope is opaque and thus considered a variety of jasper (see below).

The high content of embedded iron compounds occasionally causes the streak to be slightly colored, which is very different from all other quartz varieties. Most of the times the streak will be colorless or white, though.

Jasper can have a very homogeneous structure and be evenly colored, and is then suitable for lapidary works. Often it is unevenly colored, either irregularly banded or patchy. Some jasper varieties are valued for their peculiar color patterns.

 

Occurrence

Jasper is typically found in veins and cracks in volcanic rocks, often together with chalcedony and agate. I haven't seen any geodes filled with jasper that resemble agate geodes, so the formation of jasper seems to be restricted to veins and cracks percolated by aqueous solutions. Veins of jasper occasionally occur in igneous rocks (for example, red and yellow jasper in a granite in the Erzgebirge, Saxony, Germany).

 

Jasper Varieties

Jasper can show many different color patterns, and these have been given their own names, like "landscape jasper", "picture jasper", "poppy jasper", "leopard jasper", "ocean jasper", and so on.


10mm 
1328x832 182kb - 2656x1664 604kb
The cracks filled out by jasper often form during tectonic activity. These movements go on for a long time, the cracks will reopen again, the jasper will be shattered, and the voids in between will be filled with more jasper. This leads to the formation of  brecciated jasper, a rock that has - often repeatedly - been shattered, and is made of jasper, and sometimes chalcedony and macrocrystalline quartz of different colors. The image shows a tumbled specimen of unknown origin. Its deep red color is caused by embedded hematite.



20mm 
2208x1520 284kb - 4416x3040 887kb
Banded jasper may from when layers of fine volcanic tuff get silicified and the original banding of the tuff layers is preserved. Such jasper has a dull fracture, but it takes good polish. The specimen is from a Permian tuff at a famous German location, Gnandstein, south of Leipzig, Sachsen. Jasper from this spot has been used for lapidary art, and even if you do not speak German, if you look at the encyclopedia page of that locality at mineralienatlas.de, you can see that Gnandstein banded jasper has made it onto a stamp of the former DDR.



10mm 
1320x1048 247kb - 2640x2096 710kb
This  is a tumbled piece of poppy jasper or leopard jasper from the Northern Cape Province in South Africa, with its typical eye-like patches of different colors. Like in the former specimen, thin quartz-filled cracks run through the piece and the centers of some eyes are also formed by macrocrystalline white quartz.


Plasma  is sometimes considered a green variety of jasper, and sometimes considered as a green translucent variety of chalcedony. Because of this ambiguity, it is mentioned here, but is still presented on its own page.


Heliotrope  is a dark green, opaque jasper with small red spots rich in iron oxides. Heliotrope is also known as bloodstone, but should not be confused with hematite (Germ.: Blutstein), which is named for its blood-red streak. The deep green color can be caused by various embedded minerals of microscopic size, as like chlorite or actinolite. The green parts of heliotrope are practically opaque. The red spots are colored by embedded hematite, Fe2O3.


10mm  
1024x920 154kb - 2048x1840 460kb
Heliotrope  can be found as vein fillings in volcanic rocks. Good heliotrope comes from India (in the vicinity of Rajkot on the Kathiawar Peninsula and in Poona), and Rio Grande do Sul, Brazil.

The  image shows a rough piece of heliotrope, probably from India.


 

Locations and Specimen

Germany


20mm 
2352x1568 464kb - 4704x3136 1336kb
A cut and polished slab of jasper from Unteralpfen, Görwihl, Waldshut in the southern Black Forest, a locality known for "carnelian", agate and occasional finds of amethyst. The interesting thing about the locality is the geological environment: the specimens are found in a coarse grained triassic quartz sandstone. Grains and larger fragments of the sandstone hostrock cam also be seen in this specimen.


Japan


10mm 
2368x1568 454kb
A rounded pebble of jasper, picked up on the beach at the Nawaji Gold Mine, Izu Peninsula. It is colored by hematite, which is also found as crystals in tiny cavities in the jasper. Along the top edge of the cut and polished cross section one can see small pyrite crystals. This is a low to medium temperature vein deposit in volcanic rocks.


Madagascar


20mm  
1200x1092 253kb - 2928x2664 1071kb
Ocean jasper from the Morovato mine on the Ambolozo (Analalava?) peninsula in north-western Madagascar. The name refers to the location at the coast. It is also known as orbicular quartz, a name that makes more sense to me, as it is an agglomerate of various quartz types: light brown chalcedony, dark green orbicular spheres (spherulites) of chalcedony or jasper, white chalcedony layers on top of or underneath translucent colorless quartz crystals, and also green quartz crystals. On a specimen in an Arizona rock shop I've once seen some dark green quartz crystals, about 1-2 cm long, that would qualify as prase in a cavity inside ocean jasper, but the specimen was a bit oversized. On mineral fairs ocean jasper can frequently be seen carved as decorative balls that look a bit like the poppy jasper specimen shown above. Material from this location shows a wide range of colors, with shades of green being most common.

Similar material of more reddish colors came from Morgan Hill in California, U.S.A, (see below).


U.S.A.


20mm 
1600x2320 486kb - 3200x4640 1447kb
Perhaps the most wanted jasper of the U.S. is Morgan Hill Poppy Jasper, from Morgan Hill, south of San Jose, California. Its texture resembles that of "Ocean Jasper" from Madagascar, as it is largely composed of intergrown spherulites, but the colors are much more showy. The locality is on private ground and closed to the public.



20mm 
1362x902 171kb - 2724x1803 550kb
A  vein of brown-red jasper in volcanic rock from Ashland, Oregon.



10mm 
1400x864 140kb - 2800x1728 459kb
Red  jasper from a stream north of Lake Beryessa, California. The front has been polished. Carnelian has the same color, but is translucent.


 

Further Information, Literature, Links


Printer Friendly Version

Copyright © 2005-2013, A.C. A k h a v a n

Impressum    -    Source: http://www.quartzpage.de/jasper.html