All About Topaz:
By: Eliana Zhang
The gemstone topaz can be found throughout history even beyond jewelry use. English superstitions whispered that topaz could cure lunacy. Ancient Romans believed that topaz brought them protection while traveling. During the Middle Ages, people thought that topaz provided protection from curses and enhanced mental abilities. Such rumors combined with topaz’s stunning looks might cloud one’s views on what topaz exactly is!
Common perceptions of topaz include its status as a semi-precious stone, but it is not necessarily a stone of lesser quality than, say, diamonds. While precious stones are rare gemstones of high quality, stones of similar quality that are just more common are classified as semi-precious. Classification ultimately comes down to the rarity of the stone, since the skill of the artisan is what makes gemstones pop instead of the gemstone itself. For example, amethysts used to be considered precious when they were rare. Once huge amethyst reserves were found, they became semi-precious stones. The gemstone topaz is not of poor quality — it is just more commonplace than diamonds and rubies.
Topaz is also the hardest mineral amongst the approximately 600 different members of the silicate family, with a hardness of 8 on the Mohs hardness scale. Despite how counterintuitive it may seem at first, topaz is brittle and cleaves easily! Hardness and toughness are always inversely related. Toughness (brittleness) is the ability to absorb energy before fracture, while hardness is the measure of how hard of a force is required for the stone to be indented or scratched. For example, if you throw a steel ball at a glass pane and steel pane of the same thickness, one would have a better chance of stopping the projectile. Both panes can be manufactured to similar strength and hardness levels, but the steel pane would slow the ball down gradually, absorbing its kinetic energy. It would deform permanently but not break. The glass panel, on the other hand, would shatter, letting the projectile keep some of its kinetic energy. Topaz would shatter, letting any projectile keep its kinetic energy; this is because topaz is hard and doesn’t allow for deformation, or the absorption of kinetic energy.
Considering all this, how is topaz even formed in nature? It typically forms near quartz, but yellow quartz and topaz aren’t the same. During the last stages of igneous rock formation, there are fluorine-bearing vapors that run along fractures. The lava runs through the fluorine, and the cooled lava-fluorine mixture is what we call topaz today. It needs harsh environments to form, normally occurring in cavities, pegmatite dikes, and high-temperature veins. It also forms near the tin ore cassiterite, so not only is topaz a good gemstone for jewelry, but it can also indicate where cassiterite could be found. Like most gemstones, topaz is pushed up to the surface after forming.
Unsurprisingly, since topaz is formed below the ground, mines are needed to access them. Open-pit mines similar to those of diamond mines are used for topaz mining. As mentioned earlier, topaz is brittle and cleaves easily, so water cannons clear out debris and miners hand-pick the topaz from the rocks. Every piece of topaz has to be handled carefully lest they damage it. After the mining process, a lot of topaz have to be treated directly to make it presentable to the public. Treatments include intense heat and a metallic oxide coat to minimize flaws and enhance its natural color.
Topaz occurs in a variety of natural colors, the most well known of which would be orange, a desirable color that makes it suitable for jewelry. Topaz actually got its name from the Sanskrit word “tapas,” which means fire! Most natural topaz is colorless though. The different colors of topaz slightly influence their characteristics. For example, brown topaz loses color when exposed to strong light and blue topaz is radioactive. The price of topaz strongly depends on its color along with its quality. Blue topaz can be bought for $25 per carat. Colorless topaz, which blue topaz is produced from, sells for $8 a carat. Brown topaz is similar. On the other hand, orange topaz sells for $1000/ct. for sizes larger than 10 carats. Pink or red topaz can sell for $3500 a carat. If you want to purchase gorgeous jewelry, topaz might be the option for you!
What did you learn?
How is topaz formed?
It needs harsh environments to form, normally occurring in cavities, pegmatite dikes, and high-temperature veins. During the last stages of igneous rock formation, there are fluorine-bearing vapors that run along fractures. The lava runs through the fluorine, and the cooled lava-fluorine mixture is what we call topaz.
Why is topaz hard yet brittle?
While counterintuitive at first, it makes sense once you understand how hardness and toughness are always inversely related. Because topaz is hard, it doesn’t allow for deformation and doesn’t absorb kinetic energy. As a result, it would shatter to let the object keep its kinetic energy.
Citations:
https://www.differencebetween.com/difference-between-precious-and-vs-semi-precious-stones/
https://thediamondreserve.com/how-is-topaz-formed/
https://www.britannica.com/science/topaz
https://www.sustainablegemstones.org/supply-chain/environmental-issues
https://www.minerals.net/mineral/topaz.aspx
https://geology.com/minerals/topaz.shtml
Image credit:
https://pixabay.com/photos/jewelry-earrings-elegance-silver-2748723/
https://pixabay.com/vectors/colorful-prismatic-chromatic-1237242/
Originally published at https://www.helyx.science on November 4, 2020.
