Jewels and gemstones

From Chempedia

Natasha Filipovitch

Anisha Chandiramani

Hussein Ahmed

Kim Ahneman

Salma Hussein

What exactly are gemstones and what are their properties? To understand gems, it is necessary to first define what a gem is. "Gem minerals are hard, relatively free from cleavage, and occur as transparent crystals. Many other minerals have been used as gems, even though they do not conform to this definition."¹ This requires a definition of a mineral: "The term mineral is used in a variety of ways…As used in the geosciences, however…a mineral is a naturally occurring crystalline solid with a definite, but not necessarily fixed, chemical composition."² Once a mineral fits into these definitions, its classification as a gem is influenced by its attractiveness, durability, rarity, fashion, and size. The different combinations of these factors may determine the value of a gem is. The size of the crystal determines whether or not a mineral is classified as a gemstone; the larger the crystal is, the more the stone is worth. The carat of a diamond, for example, refers to its size, unlike karats of gold, which refer to the purity of the mineral. Many gemstone minerals require high temperatures and/or high pressures to form. Some minerals, like diamonds are formed at high pressures deep within the earth and then ejected by volcanoes. Others, like red emeralds, are created at surface pressure but high temperatures, in this case, "in excess of 300ºC."³ The visible crystal structure of a gem is caused by the internal arrangement of atoms, which is determined by the relative sizes of the cation and anion in ionic bonds. The configuration with the lowest energy will change based on these relationships, creating coordination polyhedra (figure 1). Putting many of these polyhedra of similar or different formation together constitutes the construction of a crystal (figure 2). Strong bonds within these crystal structures result in gemstones relative hardness.

Figure 1- Various Coordination Polyhedra. The positions of cations and anions shown here can also be reversed, although it is more common for the cation to be smaller than the anion. (Reference 4)

Figure 2 - Crystals with defined octahedral shape. (Reference 4)

The magnificent colors found in gemstones are largely due to selective absorbance and reflection of different wavelengths of light. Usually, a very small percentage of metallic oxides are present in the gem and determine the colors absorbed. From this very basic property of matter we know that whichever portion of the visible spectrum is absorbed by the gemstone, the complementary colors are reflected.⁵ Thus, in the brilliant sparkle of a red ruby, blue and green wavelengths of light are being absorbed the most (figure 3).

Figure 3 - A gemstone\’s absorptive and reflective properties. (Reference 7)

There are also many different optical properties that vary the displayed patterns of color in a gemstone. One such property is called Pleochroism, which appears in colored, transparent gemstones. This phenomenon occurs because the absorption of light within the gemstone is different for different angles of light, or polarization directions. Polarized light is light in which the electric vectors are emitted in a non-random fashion. Some gemstones, such as tourmaline, have the ability to completely absorb one polarized component of a light source and can act as a polarizer. A Pleocroic gem will display different colors in different angles of light.⁵ Many gemstones, commonly known as "star stones," display a property called asterism. Small, parallel fibers compact densely in the mineral, causing light to reflect in a star-like formation. Small needles of titanium oxide, called rutile, cause the star-like reflection to move around the cabochon, or rounded, surface of the gemstone. Gemstones can exhibit four, six, or even twelve point "stars," the rays of which are caused by reflections of the rutile crystals. This effect is also commonly known as "cat’s eye."⁶ The aforementioned mineral oxides, such as rutile, in a gemstone are actually impurities, "elements that are not present in the pure compound."⁹ These impurities are trace elements, and thus occur in very small amounts. Many such trace elements can occur in any one mineral, creating the possibility for many different colors in minerals and gems (figure 4). The creation and color of gemstones are just two topics of the immense and fascinating world of gemstones. Other topics that could be explored include symmetry, formation environments, hardness, durability, radioactivity, cleavage, and many more.

Figure 4 – Sapphires are one gem that come in a particularly wide variety of colors. (Reference 8)

References

1 – D.G.A. Whitten & J.R.V. Brooks "The Penguin Dictionary of Geology" © D.G.A. Whitten & J.R.V. Brooks 1972, first published 1972, reprinted 1973, 1974, 1975 pp. 198

2 – William D. Nesse "Introduction to Minerology" © 2000 Oxford University Press, Inc pp

3 – Richard V. Fisher, Grant Heiken, Jeffery B. Hulen "Volcanoes, Crucibles of Change" © 1997 Princeton University Press pp. 228

4 – What is a Crystal? <http://socrates.berkeley.edu/\~eps2/wisc/Lect4.html> accessed

5 – Richard T. Liddicoat, Jr., "Gem", in AccessScience@McGraw-Hill, < <http://www.accessscience.com/Encyclopedia/2/28/Est_284100_frameset.html?doi> last modified: January 28, 2002. accessed 9/27/05

6- http://en.wikipedia.org/wiki/Asterism_%28gemmology%29, accessed 9/27/05

7 – What is a gem? <http://ist-socrates.berkeley.edu/\~eps2/wisc/Lect2.html> accessed 9/27/05

8 – Color in minerals <> accessed 10/13/05(to access use reference 9 and click on the Causes of color in minerals link.)

9– Color in Minerals <http://socrates.berkeley.edu/\~eps2/wisc/Lect7.html> accessed 10/13/05