Guest Videos: Chocolate and Geodes

OK, so chocolate and sugar aren’t technically minerals…they’ve still powered a lot of geologists on field trips.

In nature, you don’t know what’s in a geode until you cut it open. This can be a lot of fun, along with watching your YouTube views hit the million mark! (Note: They’re getting close to two million views now, and bless them, they haven’t monetized it. Thanks!)

Guest Video: Saharan Desert Glass

Once upon a time, a shallow tropical sea called Tethys separated Africa and Eurasia, covering what is now the Sahara Desert and connecting the Atlantic and Pacific oceans with an equatorial belt of warm water.

Then, one day almost 30 million years ago, something from space burned in through the atmosphere, either striking our planet near the Sahara’s location or exploding in the air above it.

This impact, while nothing like the event that caused a major mass extinction 65 million years ago, was more powerful than an atomic explosion–strong enough to turn exposed areas of quartz-rich sand into glass.


James St. John, CC BY 2.0

Around 19 million years ago, the Arabian tectonic plate began to collide with Eurasia. Slowly, inexorably, land sealed off and isolated what had been the western arm of Tethys, turning it into the Mediterranean Sea.

For this and many other reasons, the world’s climate changed dramatically, and the Sahara became a desert.

Millions of years later, human beings who knew how to survive and prosper in this great desert came along and used this unusual material for tools and, later on, jewelry.

However, seen under a microscope, the Sahara still harbors jewel-like grains of sand.

Featured image: Western Desert sand grains. Wilson44691. Public domain.


Agustí, J., and Antón, M. 2002. Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. New York and Chichester: Columbia University Press.

Giuli, G., Paris, E., Pratesi, G., Koeberl, C., & Cipriani, C. (2003). Iron oxidation state in the Fe‐rich layer and silica matrix of Libyan desert glass: A high‐resolution XANES study. Meteoritics & Planetary Science, 38(8), 1181-1186.

Guest Video: Collecting Topaz in Utah

Topaz is the hardest silicon-based mineral, and its clarity and beautiful color make this a beautiful and very popular gemstone.

Topaz got its name from the island of Topazos in the Red Sea, which is also known as Zabargad Island–once the source of all the world’s garnets.

Besides being confused with those yellowish-green garnets, topaz used to be the name for any yellow gemstone, but today it refers to just one mineral (which comes in several colors and also happens to be Utah’s state gem).

And yes, you can go out and collect some!

Featured image: Rob Lavinsky, CC BY-SA 3.0.

Sources: Wikipedia. 2018. Topaz. Last accessed March 30, 2018.

Guest Video: Microphotographs of Gemstones

Microphotgraphy using polarized light yields beautiful art.  It also is very helpful in learning more about a gem.



Tomorrow we’ll check out a couple other ways that microscopy reveals information about places we can’t reach: the deep Earth and the past.


Metamorphic Gem: Garnet

Mantle gems include diamonds and peridots. These form under intense heat and pressure many miles below our feet.

Another type of deep-Earth gem forms closer to the surface, where metamorphism takes place .

Those garnets look a lot like the ones I used to search for in the Berkshire Hills of western Massachusetts, where I grew up (and where I once stumbled across “gold“). Rocks there are layered like lasagna.

Most garnets there and elsewhere are industrial grade–deep red clear to opaque crystals that wouldn’t make you rich when collected for fun but are profitable when mined in bulk for industrial use as abrasives.

Garnets can come in every color, including blue, which was once thought to be impossible. Garnet translucency is such that even a small amount of a minor element can make a big difference.

Metamorphic gems like garnet don’t look expensive until they have passed through a gemcutter’s hands.

Green garnets are the most valuable:

  • Russian tsars liked demantoid garnet, which can outsparkle both diamond and emerald.
  • Tsavorite garnets rival emeralds in color and usually have fewer inclusions, but they are usually small and common. Only large stones (2 carats or more) can command super-high prices.

Even uncut tsavorite can be beautiful. (Rob Lavinsky, CC BY-SA 3.0)

Technically, garnets are a mineral group, not a specific gemstone. And that is not surprising, given the wide assortment of mineral-forming elements there are underground and all the high-pressure, high-heat geologic drama that accompanies garnet formation!

Thanks to plate tectonics and subsequent weathering, which uncovers underground rocks, garnets occur literally everywhere from Afghanistan to Zambia.

Just keep an eye on the blue ones–they’re tricky.

Update, May 19, 2018: Some researchers have discovered that garnets also might be helping keep Earth’s surface from resembling that of Mars!

Featured image: Rob Lavinsky, CC BY-SA 3.0.

Barnfield, J., and Cook-Wallace, H. 2002. Gems and Gem Material. University of California Berkeley, Department of Earth and Planetary Science. Last accessed March 17, 2018.

Collectors Weekly. n.d. Antique and Vintage Garnet Jewelry. Last accessed March 17, 2018.

Karabinos, P.; Morris, D.; Hamilton, M.; and Rayner, N. 2008. Age, origin, and tectonic significance of Mesoproterozoic and Silurian felsic sills in the Berkshire massif, Massachusetts. American Journal of Science. 308(6), 787-812.

Levin, S. B. 1950. Genesis of some Adirondack garnet deposits. Geological Society of America Bulletin. 61(6): 519-565.

Stoffer, P. 2017. Gems from metamorphic rock. Geology of Gems, Chapter 10. Last accessed March 17, 2018.

United States Geological Survey. 2016. Garnet, in “An Overview of Production of Specific US Gemstones.” US Bureau of Mines Special Publication 14-95. Last accessed March 17, 2018.

University of Minnesota Department of Geology. n.d. Garnet. Last accessed March 17, 2018.

Zabargad: Island of Green Stones

What do former Russian royals and the Red Sea have in common?

Gems, of course–in particular, one famous peridot plus a few smaller ones. All of these stones came from Zabargad Island in the Red Sea.

It’s not easy to find a color image of the royal Russian chrysolite (an old name for peridot), but it looks majestic even in black and white.


This transparent olive-green peridot is 2 inches long, 1.4 inches wide, and weighs over an ounce (196 carats). It now sits in the Kremlin together with six other historic royal gems. Source: A. E. Fersman.

Although you can’t see it here, the gemstone is set in silver and gold, surrounded by 30 diamonds. That setting was made early in the 19th century, but sometimes the enormous chrysolite is exhibited alone. Unlike most other peridots, it is close to perfect, with only three microscopic cracks!

Probably Romanov ladies wore it as a brooch or locket.

The lore handed down about this Russian crown jewel was that some crusaders had brought it out of the Holy Land. However, in 1900, gem experts connected it to Zabargad Island in the Red Sea.

Today Zabargad – a little piece of mostly barren land in the mdist of sparkling warm water – is a popular diving site. Back in the days of the pharaohs, though, it was the only place in the known world where you could mine peridot.

While ancient Egyptians certainly had true emeralds, some of their green stones actually may have been peridots. Anyway, the pharoahs kept such close control over Zabargad that any unauthorized visit could easily earn you the death penalty.

The Roman naturalist Pliny the Elder was familiar with this famous mining site, which was called Topazios back then, as well as less glittery names, like the Island of Death and Snake Island.

During the Crusades it became known as St. John’s Island and then Zabargad–Island of Green Stones.

I don’t speak Italian, but Zabargad is such a popular diving spot now that it’s not easy to find videos online that focus on the land. (If you understand the narration, feel free to share it in a comment.)

Although Arizona and Norway are the go-to places for today’s peridot miners, Zabargad is still an island of green rocks because of its geology.

As we saw recently, peridot is a mantle gem and this island was originally part of Earth’s upper mantle.

Its peridotite and gems were probably uplifted when the Red Sea spreading center between the African and Arabian plates first opened up roughly 35 million years ago.

However, the green rocks of Zabargad Island may record events that happened deep underground 600-700 million years ago, when this region might have been a subduction zone and the supercontinent of Gondawanaland was taking shape.

Either way, that’s a lot of geologic drama to produce such a quietly elegant gemstone.


Gini. CC BY-SA 2.0.


Gübelin, E. 1981. Zabargad: The ancient peridot island in the Red Sea. Gems and Gemology. 17(1): 2-8.

Revheim, O. 2015. Peridot from St. John’s/Zabargad Island. Last accessed March 8, 2018.

Guest Video: What’s Cooking? Montana Sapphires!

We’ve looked before at how sapphires form. Now let’s check out Montana sapphires – North America’s biggest known deposit of gem-quality stones – and see how gemologists manage to get every bit of color they possibly can from each stone.

Even the green ones!

Featured image: Pumpkin Sky, CC BY-SA 3.0.

Guest Video: Peridot, A Mantle Gem

Did you know that many diamonds form underneath continents on a “mantle keel”?

These beauties aren’t the only mantle-associated gems.

Most of the upper mantle is made out of peridotite, a combination of olivine and pyroxene. It is named after the very famous gemstone it contains–peridot.

Combined with diamonds, peridot made a beautiful addition to early 19th century Habsburg royal jewels.

Peridot usually reaches the surface through volcanism, but this gemstone mineral has also been found in meteorites and even in comet dust.

The space connection isn’t surprising when you consider that the mantle makes up over 80% of our planet, which formed out of a space dust cloud.

Here are some people in Australia collecting peridot that had a “homey” origin in our planet’s mantle.

Featured image: Aomai CC BY-SA 3.0.