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.

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.

A List of the Top Five Diamonds

This was first posted in my Clear Sight blog on November 22, 2014; I’ve edited it a bit.



How would you choose the world’s top five diamonds? By cost? Carat? Who owns them?

People occasionally pay insanely high prices for a stone, but a truly historic diamond’s monetary value also depends on social factors that change over time.

That’s why you can’t go by price. Wealthy people certainly pay for excellent quality, but they also buy gems to impress other people or to fulfill some other inner need; or to possess something that was once owned by somebody they admire; or for historic value; or for a multitude of other reasons.

Jewelers, of course, cater to social factors, but at the business level they have to judge a diamond objectively.

They go by:

  • Its chemical structure (type)
  • Internal and external imperfections (clarity)
  • Its hue, tone, and saturation (color)
  • How well it was cut into a gem (cut)
  • Its weight (carat)

You might already have heard about the 4 C’s – clarity, color, cut and size.

But what is diamond type all about?

A perfect diamond crystal structure doesn't exist in nature.   (Source)

A perfect diamond crystal structure like this doesn’t exist in nature. (Source)

Type I and type II diamonds

The perfect diamond is a crystal of pure carbon. However, diamonds form under natural conditions of high pressure and temperature and can never be completely pure. Nitrogen and boron are the impurities we’re going to look at for diamond type.

Most transparent and colored diamonds are type I – they contain a measurable amount of nitrogen.

In simplified terms, type Ia stones have the nitrogen atoms sitting close to each other. This affects the diamond’s clarity and color.

Type Ib diamonds also contain nitrogen, but the atoms are further apart. This type is rare and often has a bright canary yellow color.

Type II diamonds contain very little nitrogen or any other impurity except boron. There are two subtypes.

Type IIa diamonds are almost pure. They make up 2% or less of all natural diamonds and will be either colorless or very lightly colored. Four of the diamonds on our list today are type IIa.

Type IIb diamonds contain enough boron to turn them blue or bluish-gray. You’d think that would make these stones less valuable, but their rarity makes them extremely precious. Only about 0.1% of all natural diamonds are type IIb, and we’ll be looking at one of them below.

Now that we’ve covered the judging criteria, here’s my list of the world’s top five diamonds:

5. The Krupp diamond

This 33.19-carat (6.638-g) type IIa diamond used to belong to the Krupp family of German industrialists. It was stolen from Vera Krupp in 1959 but recovered. It may be internally flawless, but officially some very slight inclusions have been noted.

Today the stone is known as the Elizabeth Taylor Diamond.

For obvious reasons.  Source

For obvious reasons. Source

This gem is a good example of how price varies for the same stone. Richard Taylor paid over $300,000 for it at an auction (a record at the time) and gave it to Miss Taylor in 1968. It was her favorite jewel, and during the estate sale in 2011 after her death, a South Korean business group paid almost $9,000,000 for it. Is the stone actually worth that much, or is much of that price added for the Elizabeth Taylor brand?

4. The Millennium Star

Why, oh, why didn’t they name this the Millennium Falcon and cut it accordingly?

Flawless and weighing 777 carats (5.5 ounces), this type IIa diamond was found in a stream bed on the property of Zaire’s Mbuji-Mayi Mine in 1990. It was the height of the Zaire’s civil war, and De Beers bought the huge crystal.

It took three years for experts at the Steinmetz Group to laser-cut it into a pear-shaped brilliant weighing 203.04 carats (1.4 ounces). That’s jeweler jargon for this:

The millennium referred to is Y2K, but in 1999 the Millennium Star first went on display at London’s Millennium Dome, so there’s that, too.

The Star was the featured gem, and De Beers had it insured for £100,000,000. Of course somebody tried to steal it. The Metropolitan Police caught the thieves before they could make their getaway. Kris Hollington’s Diamond Geezers is about this attempted robbery and it also includes a detailed history of the Star.

The Millennium Star is only the second-largest cut diamond in the world today. The biggest one is…

3. The Centenary

In 1988 – the 100th anniversary of De Beers – a rough type IIa diamond weighing almost 600 carats (4.2 ounces) was found at the company’s Premier Mine in South Africa. Of course they named it the Centenary diamond.

The huge stone was flawless and had the top color grade of D. It was very carefully cut into a 273.85-carat (1.9-ounce) modified heart-shaped brilliant with 247 facets.

The Centenary diamond was insured for over $1,000,000 when it went on display in 1991. De Beers loaned it to the Tower of London for a few years, but this valuable gem is now in private and understandably anonymous hands today.

2. British crown jewel diamonds: Cullinans I and II

In January 1905, the Premier Mine’s manager found a type IIa diamond weighing over 3,000 carats (1.3 pounds). It was, and still is, the largest gem-quality diamond ever found. The media christened it the Cullinan diamond, after the mine’s owner Thomas Cullinan, and the name stuck.


Source (Image edited by BJD)

The South African legislature bought the Cullinan diamond and presented it to Britain’s King Edward in 1907 for his birthday.

In 1908, I. J. Asscher cut the huge crystal ino nine major stones, 96 smaller gemstones, and 9.5 carats of unpolished fragments.

Of the nine major stones, the ones we’re interested in here are Cullinan I and Cullinan II, which ended up in the British crown jewels. The 530.2-carat (3.7-ounce)

Cullinan I – D color grade and potentially flawless – was cut into a pear shape. Now, as the Great Star of Africa, it sits on the Royal Scepter With Cross.

Cullinan II, the Lesser Star of Africa, is also D color and potentially flawless. It decorates the Imperial Crown as a 317.4-carat (2.2-ounce) cushion-shaped brilliant.

1. The Hope Diamond

This 45.52-carat gem, the biggest blue diamond in the world, is the only type IIb stone on our list. It came from Kollur, one of India’s Golconda mines. A French traveler brought it to Europe in the 17th century, where it eventually was sold to King Louis XIV. It disappeared for 20 years after the French revolution. Its history after that is a little uncertain, but this diamond eventually ended up recut and in the hands of British banker Henry Thomas Hope. Now known as the Hope Diamond, it was sold and resold repeatedly until Harry Winston bought it in 1949 and gave it to the Smithsonian Institution, where it now is displayed as part of the National Gem Collection.

The Hope Diamond.  Image by Chip Clark

The Hope Diamond is over a billion years old. Image by Chip Clark

It has been studied intensively at the Smithsonian. Here in detail is everything you might want to know about its history and science.

There you have it. It may seem odd, but I think the most valuable diamond in the world is blue, not transparent.


Featured image: Public Domain Photography, Flickr.

Why Qatari Royal Jewels Are Unique

If you’re like me, “royal jewels” only means the British crown jewels.

And you might not have known that Qatar’s emeritus ruler Hamad bin Khalifa al-Thani has apparently the world’s best collection of Mughal-era Indian jewelry until a daring jewel theft happened on the last day of the collection’s showing at the Doge Palace in Venice recently.

Apparently this collection is so unique that the thieves will find unloading the pair of earrings and the jeweled brooch they stole very difficult. (That’s not much loot, but it’s still worth millions.)

While we are all waiting for a movie to be made about this crime, let’s take a closer look at that amazing collection.
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