Rock Picture of the Week Archive
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Photo by Jim Bryan
This is a case of ordinary quartz crystals gaining special beauty and value by the addition of a mineral. What that mineral is has been debated by the experts for years. The three candidates are iron, titanium, and manganese with most agreeing on iron. In actuality it might be any or all of them. The color of amethyst varies from location to location, so it is possible that different minerals are the cause. The color is from thin layers deposited on the crystal faces, sometimes multiple layers, during their formation.
Photo by Jim Bryan
This is a coating of sugar-like Quartz (SiO2) crystals over stalactites of Chalcedony. Chalcedony is non-crystaline. Both are chemically the same, but formed at different temperatures, pressures and cooling times. Generally the slower the cooling, the larger the crystals. This specimen came from San Benito Co., California, in an area where a piece of ocean bottom was thrust up though the continent.
Photo by Jim Bryan
Clinochlore (Mg,Al)3(AlSi3O10)(OH)2- Mg3(OH)6 This is a specimen that was created in the contact zone when a chunk of ocean bottom was thrust up through the continent. The Clinochlore sticks up off the rock like little green leafs. Also mixed in, some on top and some below, are apple green Grossularite Garnets. Clinochlore belongs to the chlorite group of minerals, common in many areas.
Photo by Jim Bryan
Benitoite (BaTiSi3O9) embedded in Natrolite. Discovered in 1907, this beautiful blue gemstone is one of the rarest in the world. The only place it was mined commercially is in San Benito County, CA, and is the state gemstone. The known deposits are now mostly depleted. Its blue color probably comes from titanium. It does come in a few other colors, although they are not as common. Benitoite is very brittle and will often crack if tapped against something. The original miners ruined a lot of it by blasting.
Photo by Jim Bryan
Fish vertebrea. These were deposited in California's central valley when it was an inland sea, about three to five million years ago. They can be found scattered throughout the sediments in the western foothills where the valley floor has been uplifted and folded, exposing the various layers.
Photo by Jim Bryan
On the bottom of California's inland sea, some 3 to 5 million years ago, a group of coral found a convenient clam shell to build on and started a small colony. Specimens like this can be found scattered throughout the uplifted and tilted sedimentary layers of the western San Joaquin Valley.
Photo by Lea Beem and Jim Bryan
Jasper (SiO2) is opaque colored Chalcedony. This brecciated jasper is surrounded by clear and white Agate, giving it the appearance of floating in a liquid. Jasper is a semi-precious mineral and comes in many colors. It has been used as jewelry since pre-historic times.
Photo by Jim Bryan
Neptunite (KNa2Li(Fe, Mn)2Ti2Si8O24) is named for the Roman god of the sea. Very appropriate since most specimens are found on the contact area of the ocean bottom material (Si,Ma) and the continental land mass (Si,Al). Contact areas often allow rare minerals to interact. It appears black and opaque, but is actually red on very thin edges. A rare mineral associated with Benitoite.
Photo by Jim Bryan
Marcasite (FeS2) is mixed with Quartz (SiO2) crystals on this specimen. The Marcasite crystals are silver colored and cube-like on the right half. Also visible are red spots of Cinnabar (HgS). This came from the now closed New Idria mine in central California.
Photo by Jim Bryan
Actinolite (Ca2(Mg,Fe)5Si8O22-(OH)2) This mass of radiating prisms was formed within a piece of ocean bottom that was thrust up through the continent. The green color is from iron, and with less it grades into Tremolite. When those two get together in a tough compact mass it can become Nephrite, a true Jade.
Photo by Jim Bryan
Serpentine (Mg6Si4O10(OH)8)
This
polished
sphere
(approx.
2.5
in. in diameter) was created
by hand using grit
and a rotating pipe. The easy way is to use a sphere making machine
designed for this
purpose.
Serpentine is very common in igneous and metamorphic rocks where water
is present during
formation.
Photo by Jim Bryan
Cerussite (PbCO3) It is a bit surprising that these large petal-like, translucent crystals are actually an ore of lead. It is often confused with minerals from the Aragonite and Barite groups.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
Pyrite (FeS2)
Commonly
called
"Fool's Gold" because of it's brassy yellow color, it is easily
distinguished
from gold by its hardness and lack of malleability. On the other hand,
it is often an ore
source for gold.
It is very common and can be found in many different
types
of rock
formations.
This specimen has cubes measuring more than 2 inches per side.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
Dioptase (CuSiO3•H2O) is a rare copper mineral. Although its striking color and clarity give it the appearance of an Emerald, its softness and fragility generally prevent its use in jewelry.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
Fluorite (CaF2) is fairly common and occurs in many different types of rock formations. It is used in the steel, glass and chemical industries. Although it comes in many colors, and often has good clarity, it is rarely used in jewelry because of its softness.
Photo by Jim Bryan
Smokey Quartz (SiO2),
is
also
known as
Cairngorm or sometimes, improperly, called Smokey Topaz. It is the
national gem of
Scotland. The color is caused by impurities in the Quartz crystal.
These crystals were formed inside a vug (hollow
cavity) in
a Quartz vein that
later collapsed, breaking most of them.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
Elbaite, (Na(Li,Al)3Al6Si6O18(BO3)3(OH)4,
belongs
to
the
Tourmaline
group
of
minerals. The name comes from Elba,
Italy where it was
discovered.
It comes in many different colors, sometimes in the same crystal. Some
will change color
depending on the viewing angle. It is harder than Quartz and often used
in jewelry.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
Ludlamite, ((Fe,Mg,Mn)3(PO4)2•4H2O), was named after Henry Ludlam, an English mineral collector from the late 1800s. Its beautiful green crystals make attractive specimens, but are too soft for jewelry. It is fairly rare and found mostly in mines.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
The Fricot nugget (pronounced Free-co), weighing 13.8 pounds, is the largest surviving crystalline gold nugget from the gold rush era. It was found at a mine near Spanish Dry Diggin's on the American River in 1865. Thought to be lost for many years, it was re-discovered in a safe deposit box in 1943.
Photo by Jim Bryan
This specimen is on display at the California State Mining and Mineral Museum in Mariposa, CA.
Galena (PbS) is the most common ore of
the
metal lead. It
is also an ore of silver due to impurities in the Galena. In this
specimen the crystals
have formed on Dolomite.
Due to its toxicity, the use of lead in areas that
may
cause contamination is
continually being reduced. The major use of lead today is in car
batteries.