Zawadi sapphire A recently found (2010) opaque variety of Corundum displays a beautiful golden bronzey sheen. Its moniker comes from the Swahili word for 'gift of the Earth'. Only found in Kenya so far near its border with Somalia, the stones have a chocolate brown colour with a shimmering moving glint caused by inclusions of rutile and haematite inside the stone (see http://on.fb.me/1EoGpBi for a detailed explanation). In order to display the optical effect they have to be cut in a unique way, as flat slab like stones with some depth, unlike the normal domed cabochons typically used for star stones (see http://on.fb.me/1PCk2bT). The original pocket is supposed to be depleted, though I'm sure that the miners are eagerly searching for more. Unlike most corundums, these stones are not treated. Loz . Image credit: Gem Adventurer

Vanadinite flowers on barite stamens We have covered this mineral before (seehttp://tinyurl.com/lny2nhb) but wanted to share this wonderful specimen displaying excellent hexagonal habit and deep red hue. The size is 6x4.5x3cm, and it was mined at the Coud'a mine in Morocco. Loz Image credit: http://www.rosellminerals.com/

Petosky Stone

Michigan's stage stone is a lovely fossil coral dating from the late Devonian to early Carboniferous that was spread around parts of the state much more recently by grinding ice sheets during the recent ice ages, often as eroded rounded pebbles. Hexagonaria percarinata (named for the hexagonal patterns) belongs to an extinct group of corals called rugose that characterise reef building organisms of the Paleozoic. In those days what is now Michigan was the floor of a shallow equatorial sea filled with coral reefs.

In some areas then entire head of fossilised colonies transformed into limestone can be discovered in the Traverse group strata, though most are recovered on the beaches of Lake Michigan, where annual frosts heave and turn stones each winter revealing new specimens. A polish on the lapidary's bench is necessary to reveal the patterns in their full glory. Each hexagon was the home of a polyp that lived within the colony, who then fed via the tentacles that emerged from the central mouth. A small number of specimens have a rosy hue due traces of iron that infiltrated during the process of the reef turning into rock (called lithification or diagenesis).

The common name comes from that of an Ottawa chief called Pet O Saga (meaning rising sun), reputedly half French, who established himself as a fur trader in the last quarter of the 19th century. The stone has been a popular souvenir of the area since the Victorian era.

Loz

Image credit: Cobalt 123

http://www.petoskeyarea.com/petoskey-stone-73/ http://www.michigan.gov/documents/deq/ogs-gimdl-GGPS_263213_7.pdf http://web2.geo.msu.edu/geogmich/petoskystone.html

Calcite wearing a hat. This incongruous crystal grew with a pseudohexagonal hat on its tip....ain't the mineral world wonderful? Loz Image credit: Crazy Dave Green

Trapiche Sapphire

Most trapiche stones are Colombian emeralds (see http://on.fb.me/1GHHKnQ), though other minerals such as iolite (aka cordierite, or in this incarnation cherry blossom stone, see http://on.fb.me/1RJiJsF) also crystallise in this unusual configuration. Corundum (both sapphire and ruby) also produces the occasional specimen like this lovely hundred carat beauty from Burma seen here (measuring 35.47 x 31.89 x 6.70mm). As you can observe, the crystal has grown in a radial manner around an older grey corundum core, with the iron and titanium that pass an electron back and forth to create the blue colour being selectively distributed and separated by bands of colourless to light yellowish corundum.

These stones are not to be confused with star sapphires or rubies (see http://on.fb.me/1PCk2bTand http://on.fb.me/1EoGpBi), which are due to light being reflected off fine needle shaped inclusions in the stone, also orientated following the mineral's hexagonal crystal structure. Whether caused by inclusions or colour zoning, these stones reveal well how crystals grow and the invisible rules of their formation that causes atoms to organise themselves in regular repeating lattices. Water crystals grow in this system too, and the diverse shapes of snowflakes are another example of this crystalline structure revealed.

Loz

Image credit: Bonhams No automatic alt text available.

dusted77• Aquamarine w/ Black Tourmaline • ~ Erongo Mts., Namibia •

Vanadinite These beautiful orange crystals are the main source of Vanadium and a minor source of lead, being a lead-vanadium chloride within the apatite group (see https://www.facebook.com/TheEarthStory/posts/1081644598563250:0). They are rare, only forming when lead ore deposits such as galena are oxidised, usually in arid climates, as circulating fluids alter the original rocks forming secondary minerals. The vanadium is leached from the wall rocks, and mixed with the lead before precipitation. Crystallising in the hexagonal system, they usually have a six sided shape. It is both brittle and dense, and only has a hardness of 4 (same as a copper coin) on the Mohs scale. It gradually darkens on exposure to light, so specimens should be kept in a dark cabinet. Colour ranges from red through orange to brown Morocco is the best source for spectacular mineral specimens like this one, but it also occurs in Europe (Austria, Spain, Scotland), Africa (South Africa, Namibia) and the Americas (USA, Argentina). Discovered in Mexico in 1801, and originally named brown lead, their analysis led to the discovery of the element Vanadium. The mineral has also been known as Johnsonite. Loz Image credit: Rob Lavinsky/iRocks.com http://webmineral.com/data/Vanadinite.shtml http://www.mindat.org/min-4139.html http://www.minerals.net/mineral/vanadinite.aspx http://www.galleries.com/Vanadinite

Parisite

Here we have one of the rarest carbonates, named not after the city in France but a mine manager at the Muzo emerald mine in Colombia where the mineral was first described. Containing rare earth elements such as cerium, lanthanum or neodymium it tends to form barrel shaped hexagonal crystals with horizontal striations. They tend to exhibit basal cleavage, a plane of weakness in the crystal where less atomic bonds are holding it together and along which they tend to preferentially split. Colour is usually beige, reddish brown or yellow and the hardness is 4.5 on Mohs scale, though pieces are only occasionally faceted for collectors. Other localities include the United States, Canada, Norway, France, Italy, Wales, Russia, Malawi, Madagascar, the Democratic Republic of the Congo, Japan, China, and Vietnam. Other than its type geological setting of bituminous metamorphosed limestones it is usually formed in pegmatites, rare earth carbonatites (carbonate magmas) and alkaline magmas. The large 6.5 x 4.5 x 4.3 cm crystal cluster in the photo was mined in Malawi

Loz

Image credit: Rob Lavinsky/iRocks.com

Purple Sapphire, Tucson Gem Show

Let it snow, let it snow, let it snow!

There are few things more beautiful than the intricacy of a snowflake, but how exactly are they formed? Why are they symmetrical? Why are no two the same?

Let’s find out!

The birth of a snowflake:

Snowflakes form in cold weather when water droplets in the atmosphere freeze onto a particulate; perhaps pollen or some dust. As the particle has now gained a bit of weight (don’t we all over the holidays?), it begins to fall towards the ground. As the ice crystal is falling, water vapour freezes onto it, building new crystals and laying the foundation for a snow flake to form. The temperature at which a crystal forms and the humidity of the air determines the basic shape of the ice crystal. For example, we tend to see long needle-like crystals at -2 degrees C and very flat plate-like crystals at -15 degrees C.

Symmetry!

Many snowflakes are symmetrical; this is due to the crystallisation of water molecules. When water molecules are in a solid state, such as in ice and snow, they form weak bonds (called hydrogen bonds) with one another. This process means that they arrange themselves in predetermined spaces which results in the symmetrical, hexagonal shape of the snowflake.

No two the same?

If you take into account the number of water molecules and isotopes of oxygen and hydrogen, then it is safe to say that no two snowflakes are identical. However, it is possible to find two that look pretty similar, but even this is a huge improbability- I definitely wouldn't recommend spending your life trying to find snowflake twins!

When you think about it, each snowflake will follow a slightly different path through the atmosphere and will encounter different conditions along its journey; so they all tend to look unique- more diversity to awe at!

-Jean

Images courtesy of Alexey Kljatov

Gorgeous cut emerald

Devil’s Postpile

Columnar joints form by thermal contraction: solids expand when they are heated and cool when they contract. Solid rock doesn’t easily change its volume, so if a rock cools a lot, the shrinkage can cause it to fracture.

Lava flows cool a lot. They start off over a thousand degrees centigrade and cool to the Earth’s surface temperature. If the rock cools at the right rate and cannot relieve stress, it will fracture and form columns. The natural shape of those columns is hexagonal, but perfect hexagonal columns are rare since rocks don’t cool perfectly – crystals inside the lava create boundaries that fracture things into different shapes. If you’re ever walking on columnar joints, do watch for whether or not they are hexagonal – very few actually are.

Columnar joints form perpendicular to the cooling direction. A simple lava flow that is deposited on the planet’s surface will cool from the bottom and the top, forming vertical columns.

Devils Postpile is a neat set of columnar joints. It formed from a lava flow in the mountains of California during the Pleistocene, a time when the mountains grew large valley glaciers. This lava flow moved downslope until it hit something solid in the way – in this case a glacier. That block of ice caused this lava flow to cool partially from the side, so the columns here twist.

-JBB

Image credit: Kurt Repanshek https://flic.kr/p/7NMGb2_ _