,
British Museum Jasper is an
opaque rock of virtually any colour stemming from the mineral content of the original sediments or ash. Patterns arise during the consolidation process forming flow and depositional patterns in the original silica-rich
sediment or
volcanic ash.
Hydrothermal circulation is generally thought to be required in the formation of jasper. Jasper can be modified by the diffusion of minerals along discontinuities providing the appearance of vegetative growth, i.e.,
dendritic. The original materials are often fractured and/or distorted, after deposition, into diverse patterns, which are later filled in with other colorful minerals. Weathering, with time, will create intensely colored superficial rinds. The classification and naming of jasper varieties presents a challenge. Terms attributed to various well-defined materials includes the geographic locality where it is found, sometimes quite restricted such as "Bruneau" (a canyon) and "Lahontan" (a lake), rivers and even individual mountains; many are fanciful, such as "forest fire" or "rainbow", while others are descriptive, such as "autumn" or "porcelain". A few are designated by the place of origin such as a brown Egyptian or red African.
Banded iron formations Jasper is the main component in the silica-rich parts of
banded iron formations (BIFs) which indicate low, but present, amounts of dissolved oxygen in the water such as during the
Great Oxidation Event or
Snowball Earth. The red bands are microcrystalline red chert, also called jasper.
Picture jaspers ) Picture jaspers exhibit combinations of patterns resulting in what appear to be scenes or images, when seen on a cut section. Such patterns include banding from flow or depositional patterns (from water or wind), as well as dendritic or color variations. Diffusion from a center produces a distinctive
orbicular appearance, i.e., leopard skin jasper or linear banding from a fracture as seen in
liesegang jasper. Healed, fragmented rock produces
brecciated (broken) jasper. While these "picture jaspers" occur all over the world, specific colors or patterns are unique to the geographic region from which they originate. One source of the stone is
Indonesia, especially in
Purbalingga district. From the US,
Oregon's
Biggs jasper and
Idaho's
Bruneau jasper from the
Bruneau River canyon are particularly fine examples. Other examples can be seen at
Ynys Llanddwyn in
Wales. A blue-green jasper occurs in a deposit at Ettutkan Mountain,
Staryi Sibay,
Bashkortostan, Russia. (The town of Sibay, in the far south of the
Ural Mountains, near the border with
Kazakhstan, is noted for its colossal, open-cast
copper mine.)
Basanite and other types of touchstone Basanite is a deep velvety-black variety of amorphous quartz, of a slightly tougher and finer grain than jasper, and less splintery than hornstone. It was the
Lydian stone or
touchstone of the ancients. It is mentioned and its use described in the writings of
Bacchylides about 450 BC, and was also described by
Theophrastus in his book
On Stones (
Ancient Greek title: :
Peri Lithon), a century later. It is evident that the touchstone that
Pliny had in mind when he wrote about it was merely a dense variety of
basalt. Basanite (not to be confused with
bassanite),
Lydian stone, and
radiolarite (a.k.a. lydite or flinty slate) are terms used to refer to several types of black, jasper-like rock (also including
tuffs,
cherts and
siltstones) which are dense, fine-grained and flinty / cherty in texture and found in a number of localities. The "Lydian Stone" known to the
Ancient Greeks is named for the ancient kingdom of
Lydia in what is now western
Turkey. A similar rock type occurs in
New England. Such rock types have long been used for the making of
touchstones to test the purity of
precious metal alloys, because they are hard enough to scratch such metals, which, if drawn (scraped) across them, show to advantage their metallic
streaks of various (diagnostic) colours, against the dark background. There are two distinct materials known as basanite: one is a black variety of jasper, while the other is a black volcanic rock closely related to basalt. Furthermore, various fine-grained black stones have historically been utilised as touchstones. Given this overlap in nomenclature and physical appearance, there is significant potential for ambiguity within the fields of petrology and mineralogy. There is an alkaline rich mafic igneous rock with the name
Basanite. ==Gallery==