Life in crystals: The science of crystal gems

A photograph of a person surrounded by giant gypsum crystals in the Cave of the Crystals in Naica, Mexico.

Image credit: Alexander Van Driessche/CC-A.

First published on 26th February 2017. Last updated 1 January 2020 by Dr Helen Klus

1. Minerals and crystals ↑

Crystals are objects with atoms that are arranged periodically. This can be seen on a large scale as they form natural cubes, triangles, or more complex symmetrical shapes like snowflakes. Many crystals are minerals.

Minerals are naturally occurring solids with a potentially crystalline structure that are not made by life forms. They are made from single elements or molecules that form a repeating pattern. New atoms attach in such a way that the pattern is repeated on every scale, making them natural fractals[1].

In this way, all minerals are crystals. To form a crystal shape, however, minerals need room to grow. Otherwise, atoms can be forced into other configurations, breaking the pattern. Rocks form when different minerals are mixed together, losing their crystalline pattern.

Crystalline minerals are generally considered beautiful. They are also rare and can be expensive. Crystalline minerals that are used for decoration, particularly in jewellery, are known as crystalline gemstones[2].

Gemstones that are not made of crystals are known as amorphous gemstones. These include rocks like lapis lazuli (made of a mixture of minerals, including lazurite), mineraloids like opal (made of non-crystalline silicon dioxide), jet (mostly carbon, made from decaying wood under extreme pressures), pearl (calcium carbonate, made from shelled molluscs like oysters), and amber (made from different materials in tree resin)[3].

The most expensive and rarest gemstones are known as ‘precious' gemstones, and These are diamonds, rubies, sapphires, and emeralds. Other gemstones are known as ‘semiprecious'[4a].

A photograph of an amethyst crystal.

Amethyst, a type of quartz crystal. Image credit: Sander van der Wel/CC-SA.

A photograph of a pyrite crystal.

Pyrite crystal.
Image credit: CarlesMillan/CC-SA.

2. How are crystal gemstones formed? ↑

Most crystalline gemstones are formed below the Earth's surface and are brought to the surface through mining or through natural processes like volcanism[5a].

Crystalline gemstones can form when water dissolves minerals, just as salt crystals form when seawater evaporates, or when magma, which contains a mixture of elements, cools[4b].

Minerals can be made of single elements like gold, silver, platinum, carbon, bismuth, iron, sodium, or potassium, where gold, silver, and platinum are known as precious metals. Other minerals include silicates (emerald, quartz and peridot), oxides (ruby and sapphire), carbonates (malachite and dolomite), phosphates (turquoise), borates (cahnite), sulfides (pyrite), sulfates (gypsum), and halides (rock salt)[6].

Some silica-based crystals like quartz can form when water dissolves silica-rich rocks, like sandstone. Other types of silicate crystals like emeralds, which are made of a mineral containing beryllium known as beryl, can form from cooling magma. As can rubies, which are made of corundum, a mineral containing aluminium oxide and chromium[5b].

Other crystals can form from heat and pressure. The most common material in the Earth's upper mantle is olivine, which is a magnesium iron silicate. In its crystalline form, olivine is known as peridot. Diamonds, which are made of carbon, can be found deeper in the mantle and are formed about 160 km (100 miles) below the Earth's surface. Diamonds can also be formed when meteorites containing graphite strike the Earth[7].

Most crystals are small, due to their confined environment. However, some crystals can grow to be extremely large, such as the giant crystals found in the Cave of the Crystals in Naica, Mexico. These are about 13 metres (42 feet) long and are mostly made of gypsum, which is a sulfate[8]. These began to form after water dissolved the mineral anhydrite, which was created from magma, and have been growing for tens of thousands of years[9].

Credit: BBC Two.

3. Crystal gemstones from space ↑

Minerals can also be found throughout space. Peridot[10a] and quartz-like crystals (cristobalite and tridymite)[11] have been found in planetary discs, which are found in young stellar systems, before planets form out of the disc's material. These are thought to be formed from shock waves in the disc that cause it to be quickly heated and then cooled.

Peridot has also been found in other galaxies[10b], orbiting brown dwarfs[12], inside of meteorites[13], comets[10c], and asteroids[14a], on the Moon[15], on Mars[14b], and falling onto newly-formed stars like rain[10d].

Rock salt crystals[16] and sand dunes made of gypsum[17] – the material that makes up the giant crystals in the Cave of the Crystals – have also been found on Mars.

Ice crystals are ubiquitous and are found almost anywhere there is ice. Water ice crystals have been found on the Moon for example[18], and methane ice crystals have been found in the atmosphere of Neptune[19].

Nanodiamonds are also fairly common and are thought to exist in the atmosphere of most stars, including the Sun. They can also be found in asteroids and meteors[20][21].

An image of peridot falling on a star.

Artist's impression of peridot falling onto a star like rain. Image credit: NASA/JPL-Caltech/University of Toledo/Public domain.

4. Properties of crystal gemstones ↑

All of the properties of crystalline gemstones are determined by the atoms, molecules, and ions that they are made of. This includes their shape, hardness, colour, and lustre[22a].

There is no scientific evidence that crystals can be used in healing. This is a type of pseudoscience[23].

4.1 Shape ↑

Crystals can be described as ‘growing', even though they are not alive. They grow as more atoms are added, and it is easier for atoms to be added in certain places than in others. This means that they form the same large-scale shape as they do on an atomic scale. It is easier to smooth out rough edges and so crystals also tend to have smooth faces[24][25].

The shape of a crystal depends on its atomic structure. Crystals tend to have atoms that are arranged in one of seven ways, known as crystal systems. These are triclinic (e.g. turquoise), monoclinic (e.g. malachite), orthorhombic (e.g. peridot), tetragonal (e.g. zircon), trigonal (e.g. ruby), hexagonal (e.g. emerald), or cubic (e.g. diamond)[26].

An illustration of atoms in an incomplete square. It is easier for atoms to bond in the incomplete sections to make a full square.

An illustration showing that it's easier for atoms to bond in certain places that form a complete shape. Image credit: Sbyrnes321/Public domain.

4.2 Hardness ↑

The hardness of a crystal can be measured with the Mohs scale. This measures substances by how resistant they are to being scratched. Talc is the softest mineral on the scale, with a Mohs value of 1, and diamond is the hardest with a Mohs value of 10. Quartz has a value of 7[22b].

Harder crystals tend to have atoms linked by covalent bonds and softer crystals tend to have ions linked by metallic bonds. Crystals with ionic bonds tend to be somewhere in the middle.

Atoms are bonded covalently if they share electrons, and this type of bond occurs between two non-metals (e.g. between different carbon atoms in diamond). Ionic bonding occurs if a metal and a non-metal exchange electrons (e.g. between the metal sodium and the non-metal chlorine in rock salt). Finally, ions are bonded metallically if the electrons are not attached to atoms, and instead are ‘free'. This generally occurs when two metals are bonded (e.g. between gold atoms)[27].

Graphite crystals, for example, are made of carbon atoms like diamonds are. However, graphite is soft with a Mohs value of less than 2. This is because all of the carbon atoms in diamonds are covalently bonded, whereas only 3 out of 4 carbon atoms are covalently bonded in graphite. Graphite is made of layers of carbon that can slide over one another and is semimetallic, containing free electrons[28].

4.3 Colour ↑

The colour of a crystal is determined by light interacting with the electrons inside of its atoms, molecules, or ions. These electrons can absorb some of the energy of the light that hits them. The light that's left is missing a specific amount of energy, and this corresponds to a missing colour. If white light is used and the red is absorbed, for example, then the crystal will look bluer. If every colour except red is absorbed, it will look red[29].

This is why sapphires look black in candlelight. Sapphires are blue when viewed in white light, like sunlight. This means they absorb most of the red light. Candlelight is mostly red, and so they absorb all of this light. There is no blue light to transmit, and so they look black.

The energy the electron takes depends on what type of atom, molecule, or ion it is a part of, and many colours are caused by impurities. Quartz, for example, is usually colourless but can be found in all colours. Amethyst is a type of quartz that looks purple due to iron, rose quartz looks pink due to titanium or manganese, and smoky quartz looks brown due to aluminium[30].

Some minerals, like diamonds, can be fluorescent, which means they glow under ultraviolet light. This happens when electrons absorb energy from the ultraviolet light, which is too energetic for us to see, and emit lower-energy light, which is visible.

Credit: via Edward Fleming.

4.4 Lustre ↑

The lustre of a crystal determines how opaque or transparent it is. When light hits an opaque material, some of it is reflected and some is absorbed. The light does not travel through it. When light hits a transparent material, very little of it is reflected or absorbed, and most travels through it.

Lustre is generally described as metallic or non-metallic, where minerals with metallic bonds generally have a metallic lustre and tend to be opaque. Non-metallic minerals can be transparent, but do not have to be. Types of non-metallic lustre include adamantine (diamond), vitreous (glass), pearly, and dull[22c].

5. Life in crystal gemstones ↑

Sometimes, microbial life can be trapped in crystals. The life forms become inanimate while the crystal continues to grow around them.

In 2000, scientists claimed to have revived life that had been dormant for about 250 million years, having formed shortly before the first dinosaurs evolved. These were found in rock salt crystals[31].

In February 2017, NASA scientists claimed to have revived life that was trapped in Naica's gypsum crystals for between 10,000 and 50,000 years. There is no natural light in Naica's caves, and so any life there must chemosynthesise rather than photosynthesise. This means that unlike most life forms on Earth, they must gain their energy from minerals rather than sunlight.

These claims are controversial because it is difficult to prove that the sample was not contaminated by contemporary life forms. However, the sterilisation involved makes this extremely unlikely.

Dr Penelope Boston, director of NASA's Astrobiology Institute, claimed that "these organisms are all very extraordinary - they are not very closely related to anything in the known genetic databases"[32][33].

Scientists hope that we can learn more about how extraterrestrial life could form by studying unusual life forms on Earth.

6. Types of crystal gemstones ↑

Mineral group

Repeating unit
 

Crystal system

Hardness (Mohs)

Colour
 

Lustre
 

Diamond

A photograph of diamond.

Single element

Carbon (C)

Cubic

10

Almost all colours

Adamantine, greasy

Ruby

A photograph of ruby.

Oxide

Corundum - aluminium oxide (Al2O3)

Trigonal

9

Red

Adamantine, vitreous, pearly

Sapphire

A photograph of sapphire.

Oxide

Corundum - aluminium oxide (Al2O3)

Trigonal

9

Generally blue

Adamantine, vitreous, pearly

Emerald

A photograph of emerald.

Silicate

Beryl - beryllium aluminium metasilicate (Be3Al2(SiO3)6)

Hexagonal

7.5-8

Green

Vitreous, sub-vitreous, waxy, greasy

Garnet

A photograph of garnet.

Silicate

Various silicate materials (X3Y2(SiO4)3). X is Ca, Mg, Fe2+, or Mn2+. Y is Al, Cr, or Fe3+.

Cubic

6.5-7.5

Almost all colours

Vitreous, sub-Vitreous, adamantine, resinous, dull

Peridot

A photograph of peridot.

Silicate

Forsterite olivine - magnesium silicate (Mg2SiO4)

Orthorhombic

7

Green to yellow, or white

Vitreous

Quartz

A photograph of quartz.

Silicate

Silicon dioxide (SiO2)

Trigonal

7

Almost all colours

Vitreous

Sugilite

A photograph of sugilite.

Silicate

Potassium, sodium, iron, lithium, silicate (KNa2Fe23+(Li3Si12)O30)

Hexagonal

6-6.5

Colourless or yellow, pink to purple

Vitreous

Topaz

A photograph of topaz.

Silicate

Aluminum fluorosilicate (Al2SiO4F2)

Orthorhombic

8

Almost all colours

Vitreous

Zircon

A photograph of zircon.

Silicate

Zirconium silicate (ZrSiO4)

Tetragonal

7.5

Almost all colours

Adamantine, vitreous, greasy

Cahnite

A photograph of cahnite.

Borate

Calcium borate arsenate (Ca2B(AsO4)(OH)4)

Tetragonal

3

Colourless to white, or brown

Vitreous, sub-vitreous, resinous

Dolomite

A photograph of dolomite.

Carbonate

Calcium magnesium carbonate (CaMg(CO3)2)

Trigonal

3.5-4

White, grey to pink

Vitreous, sub-vitreous, resinous, waxy, pearly

Malachite

A photograph of malachite.

Carbonate

Copper carbonate hydroxide (Cu2CO3(OH)2)

Monoclinic

3.5-4

Green to yellow

Adamantine, vitreous, silky, dull, earthy

Halite

A photograph of halite.

Halide

Rock salt - sodium chloride (NaCl)

Cubic

2.5

Generally colourless or white

Vitreous

Turquoise

A photograph of turquoise.

Phosphate

Copper aluminium hydroxy phosphate (CuAl6(PO4)4(OH)8·4H2O)

Triclinic

5-6

Blue to green

Sub-vitreous, resinous, waxy, dull, earthy

Gypsum

A photograph of gypsum.

Sulfate

Calcium sulfate dihydrate (CaSO4·2H2O)

Monoclinic

2

Generally colourless or white

Vitreous, sub-vitreous, silky, pearly, dull

Pyrite

A photograph of pyrite.

Sulfide

Iron sulfide (FeS2)

Cubic

6-6.5

Brass-yellow

Metallic

Data credit - mindat.org, Hudson Institute of Mineralogy, last accessed 01-06-17.
Image credit - Diamond: USGS/Public domain, Ruby: Rob Lavinsky/iRocks.com/CC-SA, Sapphire: Rob Lavinsky/iRocks.com/CC-SA, Emerald: Géry PARENT/CC-SA, Garnet: USGS/Public domain, Peridot: Rob Lavinsky/iRocks.com/CC-SA, Quartz: Didier Descouens/CC-SA, Sugilite: TIS0421/CC-SA, Topaz: Rob Lavinsky/iRocks.com/CC-SA, Zircon: Rob Lavinsky/iRocks.com/CC-SA, Cahnite: Rob Lavinsky/iRocks.com/CC-SA, Dolomite: Didier Descouens/CC-SA, Malachite: Rob Lavinsky/iRocks.com/CC-SA, Halite (Rock salt): Parent Géry/CC-SA, Turquoise: Adrian Pingstone/Public domain, Gypsum: Parent Géry/Public domain, Pyrite: Teravolt/CC-A.

7. References ↑

  1. ↑ Smithsonian Education, 'Minerals, Crystals and Gems', last accessed 01-06-17.

  2. ↑ Smithsonian Education, 'Gems: The Chosen Few', last accessed 01-06-17.

  3. ↑ Smigel, B., 'Physical Properties of Gemstones', last accessed 01-06-17.

  4. ↑ (a, b) National Geographic, 'Minerals and Gems', last accessed 01-06-17.

  5. ↑ (a, b) College of Natural Resources, UC Berkeley, 'Where do gems form?', last accessed 01-06-17.

  6. ↑ Belanger, D., UC Santa Cruz, 'Berzelian Classification System', last accessed 01-06-17.

  7. ↑ Lawrence Livermore National Laboratory, 'Shock compression research shows hexagonal diamond could serve as meteor impact marker', last accessed 01-06-17.

  8. ↑ Bernabei, T., Forti, P., and Villasuso, R., 2007, 'Sails: a new gypsum speleothem from Naica, Chihuahua, Mexico', International Journal of Speleology, 36, pp.23-30.

  9. ↑ Sanna, L., Forti, P., and Lauritzen, S. E., 2011, 'Preliminary U/Th dating and the evolution of gypsum crystals in Naica caves (Mexico)', Acta Carsologica, 40, pp.17-28.

  10. ↑ (a, b, c, d) NASA, 2011, 'Spitzer Sees Crystal Rain in Infant Star Outer Clouds', last accessed 01-06-17.

  11. ↑ NASA, 2008, 'Quartz-like Crystals Found in Planetary Disks', last accessed 01-06-17.

  12. ↑ NASA, 2005, 'NASA's Spitzer Finds Failed Stars May Succeed in Planet Business', last accessed 01-06-17.

  13. ↑ MSFC Planetary Science Group - NASA, 'Meteorites and Craters', last accessed 01-06-17.

  14. ↑ (a, b) NASA Jet Propulsion Laboratory, 2014, 'Curiosity Finds Iron Meteorite on Mars', last accessed 01-06-17.

  15. ↑ Yamamoto, S., Nakamura, R., Matsunaga, T., Ogawa, Y., Ishihara, Y., Morota, T., Hirata, N., Ohtake, M., Hiroi, T., Yokota, Y., and Haruyama, J., 2010, 'Possible mantle origin of olivine around lunar impact basins detected by SELENE', Nature Geoscience, 3, pp.533-536.

  16. ↑ NASA, 2014, 'Crystals May Have Formed in Drying Martian Lake', last accessed 01-06-17.

  17. ↑ NASA, 2011, 'NASA Mars Rover Finds Mineral Vein Deposited by Water', last accessed 01-06-17.

  18. ↑ National Space Science Data Center - NASA, 'Ice on the Moon', last accessed 01-06-17.

  19. ↑ NASA, 2011, 'Hubble's Neptune Anniversary Pictures', last accessed 01-06-17.

  20. ↑ NASA, 2014, 'NASA Scientists Find Diamonds and Other Treasures in Gold Rush Meteorite', last accessed 01-06-17.

  21. ↑ Dai, Z. R., Bradley, J. P., Joswiak, D. J., Brownlee, D. E., Hill, H. G. M., and Genge, M. J., 2002, 'Possible in situ formation of meteoritic nanodiamonds in the early solar system', Nat, 418, pp.157-159.

  22. ↑ (a, b, c) Belanger, D., UC Santa Cruz, 'Physical Characteristics of Minerals', last accessed 01-06-17.

  23. ↑ Micke, O., Schönekaes, K., Mücke, R., Kisters, K., and Büntzel, J., 2010, 'Mystical stones in oncology: crystal healing power or perfect nonsense?', Trace Elements & Electrolytes, 27, pp.73-79.

  24. ↑ Caltech, 'Crystal Faceting', last accessed 01-06-17.

  25. ↑ Sattler, K. D., 2016, 'Handbook of Nanophysics: Nanoparticles and Quantum Dots', CRC Press.

  26. ↑ Imperial College London, 'Crystal System - Rock Library', last accessed 01-06-17.

  27. ↑ The Open University, 'An introduction to minerals and rocks under the microscope: Metallic structures and bonding', last accessed 01-06-17.

  28. ↑ Partoens, B. and Peeters, F. M., 2006, 'From graphene to graphite: Electronic structure around the K point', Phys. Rev. B, 74, pp.075404.

  29. ↑ College of Natural Resources, UC Berkeley, 'Color in Minerals', last accessed 01-06-17.

  30. ↑ The Open University, 'An introduction to minerals and rocks under the microscope: Colour', last accessed 01-06-17.

  31. ↑ Vreeland, R. H., Rosenzweig, W. D., and Powers, D. W., 2000, 'Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal', Nat, 407, pp.897-900.

  32. ↑ Boston, P., 2017, 'The Astrobiological Exploration of Earth and Mars', American Association for the Advancement of Science Annual Meeting 2017.

  33. ↑ Amos, J., 2017, 'Naica's crystal caves hold long-dormant life', BBC News.

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