The Fascinating Formation of Zircons: Exploring the Origins and Colors of this Gemstone

As a December birthstone, blue zircon is a highly sought-after gemstone. But are all blue zircons natural, or do they undergo treatment to achieve that coveted hue? To understand the true nature of this captivating gem, we must delve into the fascinating world of zircon formation and the factors that influence its diverse array of colors.

Where Do Zircons Form?

Zircons are found in a variety of geological environments, including igneous, metamorphic, and sedimentary rocks. These resilient crystals form under a wide range of conditions, showcasing their adaptability and longevity.

Igneous Zircon Formation

Zircon crystals of igneous origin have an exceptionally high melting point, allowing them to form within silicic magma as it slowly cools. In this process, the mineral baddeleyite, which contains zirconium (Zr), melts and provides the necessary ingredient for zircon crystallization.

Metamorphic Zircon Formation

Under metamorphic conditions, the zirconium present in surrounding minerals like baddeleyite can leach out and recrystallize into zircon when existing rocks partially melt. This metamorphic zircon formation can result in three distinct forms: bead zircons (tiny individual crystals lined up as inclusions in other gems), polycrystalline zircons, and metamorphic zircons.

Zircon Associations

Zircons can also form in deposits that include other minerals, such as corundum (ruby and sapphire), garnet, muscovite, and spinel. These mineral associations can lead to the formation of crystals containing multiple mineral species, with zircon crystals sometimes becoming incorporated as inclusions during the growth of other gems.

Zirconium Origins

The key ingredient for zircon formation is the element zirconium (Zr), which is derived from various sources depending on the geological environment. In igneous settings, the zirconium comes from the melting of the mineral baddeleyite, while in metamorphic environments, it leaches out of surrounding minerals and recrystallizes into zircon.

Inclusions in Zircons

The inclusions found within zircon crystals can provide valuable insights into the conditions under which they were formed. Inclusions such as white mica and coesite can help scientists understand the specific metamorphic conditions that led to the zircon's crystallization.

How Do Zircons Get Their Colors?

Zircons exhibit a wide range of natural colors, from colorless to brown, red, blue, and green. These hues are primarily the result of impurities in the crystal's chemical composition and the effects of natural radiation.

Colorless Zircons

Zircon has the chemical formula ZrSiO4, consisting of zirconium, silicon, and oxygen. If a zircon forms without any impurities, it will appear colorless and can rival the brilliance of diamonds, especially due to its high dispersion.

Brown Zircons

The presence of iron (Fe) impurities in the zircon's crystal lattice is a common cause of brown or brownish coloration. These iron atoms bond with available oxygen, forming darkly colored iron oxides that give the zircon its earthy hue.

Red Zircons

Natural red color in zircon is attributed to the presence of iron and chromium (Cr) impurities. Additionally, zircons exposed to high levels of natural radiation can develop color centers, which are localized deformations in the crystal structure that absorb selective light wavelengths, resulting in red or orange colors.

Blue Zircons

Blue is the most famous and popular zircon color, but it is almost always the result of heat treatment after the gem is mined. Very rare natural blue zircon crystals may contain light-scattering inclusions that create the coveted blue hue, but the vast majority of blue zircons on the market have been heated to alter their original color, typically from darker shades like brown, brownish red, or brownish orange.

Green Zircons

Natural radiation is the primary cause of green color in mined zircons. The interaction between the zircon's crystal structure and the radioactive elements it contains can lead to the formation of color centers that absorb specific wavelengths of light, resulting in the green hue.

Zircons and Geological History

Zircons are remarkable for their ability to withstand extreme conditions and preserve a record of Earth's geological history. Their high hardness, superior chemical resistance, and high crystallization temperature allow them to survive for billions of years, making them invaluable tools for geologists.

Zircon Dating

Zircons can incorporate trace amounts of uranium (U) or thorium (Th) during their formation. These radioactive elements slowly decay into lead (Pb) over time, and by measuring the ratio of uranium or thorium to lead, scientists can accurately date the age of zircon crystals. This process provides crucial information about the timing of geological events and the evolution of the Earth's crust.

Zircon as Environmental Snapshots

As zircons grow over hundreds of millions of years, they capture "snapshots" of their surrounding environment in each growth layer. By studying the impurities and other features within these layers, geologists can gain insights into the geological conditions that prevailed at the time of each layer's formation.

The Impact of Radiation on Zircons

Zircon's physical and optical properties can vary significantly due to its exposure to radiation from uranium or thorium impurities. The greater the level of radiation, the lower the zircon's specific gravity and refractive index, and the cloudier its appearance. Highly irradiated zircons can even become virtually amorphous, losing their classic birefringence.

Interestingly, heat treatment can reverse the effects of radiation on zircons, improving their clarity. However, this process can also increase the chances of the zircon breaking, as they are already known for their very brittle tenacity.

The Fragility of Zircons

Despite their impressive durability in extreme geological conditions, zircons are notoriously fragile as gemstones. Their "very brittle" tenacity makes them highly susceptible to shattering and chipping due to impacts, in contrast with most other common gemstones, which have a "brittle" tenacity.

This brittleness is a result of the strong chemical bonds that allow zircons to persist for billions of years. While these bonds provide exceptional resistance to weathering and erosion, they also make the crystals more rigid and prone to breaking under stress.

In conclusion, the formation and properties of zircons are a fascinating study in the interplay between geological processes and the unique characteristics of this remarkable gemstone. From their diverse origins to their vibrant colors and their role in understanding Earth's history, zircons continue to captivate both scientists and gem enthusiasts alike.