Category Archives: Articles on fossils/gems/minerals

Swedish Blue

Fall of 2023 we made the trek up to Minot, ND, to visit the Nordic HostFest. While wandering around we came across a silversmith booth that specialized in “Swedish Blue”. Not knowing anything about this “stone” I took a closer look. They had raw samples for sale in a basket – and I foolishly didn’t purchase any – as well as numerous beautiful cabochons worked up in silver bezel mountings. The stone looked like a silicate, with conchoidal fracturing similar to what you might see with opal, and was streaked with varying shades of sky or stormy blues. I purchased a pendant and earrings, then later visited their website to learn more.

I highly suggest reading more about the silversmiths and stones on their page: https://www.swedishbluejewelry.com/

The trade name or gemstone name for the stone is “Swedish Blue” – especially for us English speakers that may have difficulties with Nordic dialects. However the name for the raw stone in Swedish is called “Bergslaggsten” – or stone from Bergslagen.

Beginning over 300 years ago in Sweden, the area of Bergslagen was mined heavily for iron ore. The ore was smelted in coal-fired ovens where the ore and surrounding rock was melted. When it reached a high enough temperature, a slag glaze would form at the top, which was scraped off of the metal and discarded. The slag comes in many colors, but the higher concentration of blues is what set this particular stone apart.

I was correct in my initial guess that it was high in silica – it is very glass-like, with copper giving it much of the blue-green colors. Much like volcanic glass, but from an iron foundry. The slag was discarded, and eventually grown over with local vegetation – only to be found by a Swedish goldsmith centuries later.

Campeloma

Campeloma are common freshwater snails, with an ancestry dating back to the Cretaceous of North Dakota. These gastropods (“stomach foot”) can be found at times with other snail genus, mainly Viviparus or Lioplacodes. Campeloma shells tend to be bulbous, with highly convex body whorls – the spirals that make up the shell, and about an inch long. Viviparus are about the same size as Campeloma, but the whorls are nearly flat on the outside, giving the shell a smooth cone appearance. Lioplacodes has bulbous whorls like Campeloma, but the overall cone-shape of the shell is much more pointy (like a stubby unicorn horn). Like their modern counterparts, these snails most likely lived between 3-11 years. They had a varied diet, including carrion and vegetation.

Septarian nodule (concretion)

External, weathered surface of a septarian nodule.

A “concretion” refers to a type of mineral deposit, where layers of precipitated minerals are attracted to a starting nucleus (such as shell or bone) – much like how a pearl forms. Generally this occurs in water-rich environment, where there are minimal restrictions or obstructions to the concretion growth, resulting in a round shape. Numerous concretions may start growing near each other, then merge to form larger masses.

These particular concretions contain angular cracks and cavities. The word “septarian” comes from the Latin word septum, meaning a wall or partition, referring to the cracks riddling through these concretions. It is believed the cracks are formed from shrinkage of material – where the outside deposited material is denser or hard, while the innermost material is softer, which shrinks and cracks over time. These cracks are then filled with precipitate minerals such as calcite or siderite.

Internal polished surface of septarian nodule, showing calcite and iron-rich deposits.

Septarian nodules are common in the northeastern part of North Dakota, near the Pembina Gorge. As they weather out of the ground, the softer, original concretionary material erodes faster than the cracks, giving the nodule a turtle-shell like appearance. These are often mistaken for fossils.

http://en.wikipedia.org/wiki/Concretion

Buchanan, Rex C., Tolsted, Laura L., and Swineford, Ada, 1986, Kansas Rocks and Minerals: Kansas Geological Survey, Educational Series 2, 60 p.

Quartz

Quartz is a very common mineral, with numerous shapes and colors. For as long as people have enjoyed shiny things, quartz has been used in jewelry and carvings. Different types of quartz include: chalcedony (white, lightly colored), agate (multi-colored, banded), onyx (agate with straight, consistent bands), jasper (red-brown), aventurine (chalcedony with shimmering inclusions), tiger’s eye (gold to red-brown), amethyst (purple), citrine (yellow to orange), prasiolite (light green), rose (pink), smoky (brown to gray), carnelian (red-orange), and others. The biggest difference in forms is whether the mineral is macrocrystalline (showing individual crystals), or microcrystalline (tiny crystals, visible under magnification). Transparent varieties showing good crystal forms such as amethyst or citrine would be macrocrystalline.

Quartz can be found as a component of many other rocks and minerals – granites, sandstone, and schist, for example. Crystals are generally six-sided, but commonly twin. It is a 7 on the Mohs scale, with a white streak. The chemical formula for quartz is SiO₂. As the second most common mineral found on earth (the most common being feldspar), quartz can be found world-wide.

It is piezoelectric – meaning it generates an electrical charge if put under stress. It is used as an oscillator in radios, watches, gauges, etc. Quartz sand is used to make glass, as well as a sandblasting abrasive.

http://www.mindat.org/min-3337.html

http://www.minerals.net/mineral/quartz.aspx

http://en.wikipedia.org/wiki/Quartz

Orthoceras

External shell

Orthoceras (meaning “straight horn”) is an extinct animal related to squid and octopus. Unlike tasty calamari, their shells (being in the phylum mollusca) are on the outside of their bodies, instead of the pen-bone or gladius seen today. They are often mistaken for another straight-shelled cephalopod, Baculites, however lived much earlier during the Ordovician through Triassic (488-190 mya), rather than the Cretaceous (145-65 mya). Limestone deposits containing mass die-offs of Orthoceras are common in Morocco. The ones from this location tend to be white in color, with a black background – often used in decorative carvings, bowls, and even countertops.

Their straight shells are divided horizontally by “septa”, which separate living chambers of the

animal. When it grew too large for one chamber, it would add on another. A tube running through the length of the shell, called a “siphuncle”, helped regulate water and air in the shell, allowing for both movement and buoyancy. They vary in size, from tiny centimeter long shells, to more than 6 feet long (more, if you add the tentacles!).

Polished Orthoceras, showing siphuncle and septae.

http://en.wikipedia.org/wiki/Orthoceras

http://www.fossils-facts-and-finds.com/orthoceras.html

Galena

Galena, a lead sulfide, is not only a source of lead, but of silver as well. Metal extraction is simplified by its low melting point. It is dark grey-silver in color, with an octahedral crystal structure. The metal surface can tarnish when exposed to air, darkening the color. It was named in 77AD by Pliny the Elder, from Greek “galene”, literally meaning lead ore.

Deposits can be found in England, Bulgaria, Australia, the USA, and many North African countries, among others. It is the State Mineral of Missouri and Wisconsin.

This mineral is used, and has been used, for a variety of applications. From kohl in ancient Egypt, lead shot, green glazes for pottery, and crystal radio sets. It has a variety of common names, such as “blue lead ore”, and “potter’s ore”.

The chemical formula for Galena is PbS, and it has a hardness of 2.5-3 on the Mohs scale. Being mainly lead, it feels very heavy when held. Its specific gravity is 7.4-7.6, whereas pure lead is 11.3. Pyrite, an iron sulfide, has a specific gravity of 4.9-5.2.

http://en.wikipedia.org/wiki/Galena

http://www.mindat.org/min-1641.html

http://www.minerals.net/mineral/galena.aspx

Palache, Charles, Harry Berman & Clifford Frondel (1944), The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Volume I: Elements, Sulfides, Sulfosalts, Oxides. John Wiley and Sons, Inc., New York. 7th edition, revised and enlarged, 834pp.: 200-204.

Tanzanite

Tanzanite crystal

Tanzanite

Discovered in 1967 in Tanzania, tanzanite (named by Tiffany & Co.) is one of the more recent birthstones for December (as of 2002, via the American Gem Trade Association). While the stone may be light violet in some cases, darker shades of periwinkle with hints of purple are sought after for higher gem quality. A blue variety of the gemstone zoisite, tanzanite has a Mohs harness of 6.5-7 – not particularly tough for a gemstone. Corundum (rubies and sapphires) sit at a healthy 9.0 on the Mohs scale.

Tanzanite, faceted

The blue color is caused by trace amounts of vanadium within the ziosite – much like trace elements can also cause various colors of diamonds. The blue can be enhanced and brought out in the stone by careful addition of heat. Tanzanite crystals are also pleochroic – meaning from different angles they exhibit different colors. The same crystal from one direction may look blue, and another direction look red or brown – thus faceting the stone can be challenging.

The chemical composition of tanzanite is: Ca₂Al₃(SiO₄)₃(OH)

References:

Newman, Renee. Exotic Gems: How to Identify and Buy Tanzanite, Ammolite, Rhodochrosite, Zultanite, Moonstone & Other Feldspars. International Jewelry Publications, 2010.

http://www.jtv.com/library/tanzanite-gemopedia.html

http://www.minerals.net/gemstone/tanzanite_gemstone.aspx

http://www.gia.edu/tanzanite-quality-factor

Central Dakota Gem & Mineral Society

Bismarck, ND