Step 1: Introduction to Gemology

Lesson 5

Summary
Why are some gems more common in nature than others? Gemology teaches us that crystalline gemstones need five things to develop: ingredients, temperature, pressure, time, and space. Let’s take a closer look at that first requirement.
Reading time: 2 min
phenakite with perettiite inclusions - uncommon elements uncommon gemstones

Phenakite contains the rare element beryllium. A few of these rare gemstones have been found to contain as inclusions another rarity, a recently discovered mineral, perettiite-(Y). This 0.64-ct, rectangular step-cut phenakite contains needle-like inclusions of this mineral. © The Gem Trader. Used with permission.

Since the principal source of ingredients for gems is the rocky Earth itself, knowing the abundance of the elements that make up the minerals in the Earth’s crust will help us understand why there are common and uncommon gemstones.

The Abundance of Elements in the Earth’s Crust

The following figures are estimated percentages of the Earth’s crust by weight.

The eight most common elements in the crust are:

  • Oxygen (O) 46.4%
  • Silicon (Si) 28.15%
  • Aluminum (Al) 8.23%
  • Iron (Fe) 5.63%
  • Calcium (Ca) 4.15%
  • Sodium (Na) 2.36%
  • Magnesium (Mg) 2.33%
  • Potassium (K) 2.09%

All other elements represent a mere 0.66% of the crust.

From a gemological perspective, the following uncommon elements in the Earth’s crust are of particular interest:

  • Manganese (Mn) 0.095%
  • Carbon (C) 0.02%
  • Zirconium (Zr) 0.0165%
  • Vanadium (V) 0.0135%
  • Chromium (Cr) 0.01%
  • Nickel (Ni) 0.0075%
  • Copper (Cu) 0.0055%
  • Cobalt (Co) 0.0025%
  • Lithium (Li) 0.002%
  • Beryllium (Be) 0.00028%

A World of Oxygen and Silicon

Oxygen and silicon make up more than 74% of the Earth’s crust. It’s not surprising that quartz (SiO2) is one of the most common gemstone minerals.

quartz - uncommon elements uncommon gemstones

Quartz, photo by Franco Dal Molin. Licensed under CC By 2.0.

However, oxygen and silicon are also found in some uncommon gemstones. Spodumene is a gemstone mineral composed of oxygen, silicon, aluminum, and lithium (LiAlSi2O6). Lithium makes up just two thousandths of 1% of the Earth’s crust.

spodumene - uncommon elements uncommon gemstones

Faceted spodumene from Minas Gerais, Brazil, on display at the Cantonal Museum of Geology, Lausanne, Switzerland. Photo by Sailko. Licensed under CC By 3.0.

Kunzite, a pinkish variety of spodumene, gets its color from the addition of another uncommon element, manganese.

kunzite - uncommon elements uncommon gemstones

Kunzite crystals, photo by Géry Parent. Licensed under CC By-ND 2.0.

From Emery Boards to Prized Jewels

Corundum (Al2O3) is a common mineral composed of aluminum and oxygen. You’ll frequently find it on sandpaper and nail files as emery. Gem-grade corundum, however, is considerably more rare, even rarer than diamonds. Corundum gems that include a rare trace of chromium are red in color. This red variety of corundum is more commonly known as ruby. Chromium is just one hundredth of 1% of the Earth’s crust.

faceted Burmese ruby - uncommon elements uncommon gemstones

Natural faceted Burmese ruby, 9.2 cts, heated and treated. Photo courtesy of liveauctioneers.com and Therese De Luxe.

Beryllium is less than three ten-thousandths of 1% of the Earth’s crust but it’s a primary ingredient of beryl gemstones (Be3Al2Si6O18). With the addition of chromium, another rare element, beryl becomes emerald.

emerald - uncommon elements uncommon gemstones

Rectangular step-cut emerald, 0.93 cts, Colombia. © The Gem Trader. Used with permission.

Likewise, the gem species spinel takes on a very rare cobalt blue color with the addition of cobalt, another rare element.

cobalt-blue spinel - uncommon elements uncommon gemstones

Spinel crystals on marble, Hunza Valley, Gilgit District, Northern Areas, Pakistan. © Rob Lavinsky, www.iRocks.com. Used with permission.