Gemstone Properties
The Strange, the Unique, and the Beautiful
Gemstones can have a wide variety of properties. These can range from the basics such as hardness, shape, color, and color. But it can get complex. Really. Diving deep into the gemstone world, you can find information on your favorite gemstone's crystal structure, chemical properties, optical information, and so much more scientific facts. That is not even touching upon the economics of gemstones.
But back to the fun science stuff, gemstones have some unique properties that enhance their beauty and make them stand out on the top. For example, diamonds are hard. In fact, they are the hardest mineral on the planet.
So, what properties are we going to discuss here? We will examine the seemingly simple properties, such as color and hardness, to the lesser-known, such as fluorescence, pleochroism, asterism, and labradorescence.
Color
Gemstones and minerals have a wide array of colors. When it comes to color, many alter or correct the color in some way to enhance or enrichen its beauty. But what exactly makes a stone a particular color? While we don't know some rocks and gems, their light absorption, impurities, and chemical composition are the major factors. As we will see, light can do a lot for a stone. Under certain forms of exposure to light, heat, or radiation, the stone can change color.
Hardness
Stones are tough! In terms of stones, this refers to the minerals ability to resist cuts and scratches. The most common metric for quick reference and ease of use is Mohs Scale, going from 1 to 10, with 10 being the hardest. Stones can scratch those lower than it on the scale.
Fluorescence
If you are an avid gem collector, you are likely aware of the origin of this term. If you are new, you can guess to which stone this term pays homage. If you guessed "fluorite," then you are correct! Fluorescence is named after the beloved gemstone fluorite.
Scientists observed fluorescence hundreds of years ago, but only around two hundred years ago was the early science expanded on. But what exactly is fluorescence? In short, a form of luminescence under UV light. But that's not the whole story. It occurs when some molecules react to short-wavelength light. These molecules initially have low energy, but when that light hits them, it sends them into a high energy state. This high energy state does not last for long, as they cannot store the energy for much time. Because of this, they are forced to emit light at lower energy and a longer wavelength than what it absorbed.
Pleochroism
Pleochroism means that a mineral can showcase multiple colors, but only when observed at certain angles. There are quite a few pleochroic minerals, even though most people associate this property with tourmaline and alexandrite. To be fair, both of them are absolutely stunning. The strength of the pleochroism varies much from stone to stone—for example, amethyst, peridot, and aquamarine range from low levels to medium levels. But andalusite, tanzanite, alexandrite, and tourmaline have extreme levels of pleochroism.
So, how does this work? The ultimate factor is the crystal's structure. From that, the polarization of light is the other deciding factor. This property can be advantageous when gemologists must pinpoint which stone it is precisely.
Asterism
This rare phenomenon refers to a starry pattern in a stone. Most people associate this with sapphire. There is a special kind of beauty in seeing a nice piece of sapphire with a shining star within. Well, there's a good reason why most people only think of sapphire. That's because only a few stones show this property. However, it can appear in garnets, rubies, quartzes, cat's eyes, and emeralds. Despite these known instances, it still is rare.'
Since this property is rare, those who find a stone with asterism have quite a valuable piece. Unfortunately, this has tempted a few to manufacture it. Luckily, detecting this trickery is simple with the right tools. As to why it occurs, it's from mineral impurities and inclusions.
Labradorescence
Labradorescence is most commonly known in labradorite. This optical effect is another kind of iridescence known as schiller, reflecting light from beneath the stone's surface. The formation of this effect is dependent on lamellar structure in the miscibility gap. It can occur in other stones, but this is rare.
Sources
https://www.mindat.org/glossary/fluorescence
https://www.thermofisher.com/us/en/home/life-science/cell-analysis/cell-analysis-learning-center/molecular-probes-school-of-fluorescence/fluorescence-basics/fluorescence-fundamentals/process-fluorescence.html
https://royalsocietypublishing.org/doi/10.1098/rstb.2016.0335
https://www.gemsociety.org/article/what-is-gemstone-pleochroism/
https://www.mindat.org/min-198.html
https://www.gia.edu/doc/A-New-Method-for-Imitating-Asterism.pdf
https://www.mindat.org/glossary/asterism
https://www.mindat.org/glossary/labradorescence
https://www.mindat.org/glossary/color