Let me tell you why gems are so beautiful

As early as 18,000 years ago, people found necklaces made of animal teeth and bones in the remains of the Peking Man in Zhoukoudian, Beijing. This is probably the earliest "gem products" of mankind. Gems are beautiful, rare and durable. There are more than 3,000 kinds of minerals found in nature, but only more than 230 kinds can be used as gem raw materials. There are only more than 20 kinds of medium and high-end gems in the international jewelry market. It can be seen that mineral rocks must meet certain conditions to become gems. Gems are the essence of many mineral rocks. Diamonds, sapphires and jade are classic representatives in the gem world.

Beauty is the primary condition for gemstones to have such high value. How do gemstones interact with light to produce their dazzling glow?

Fire makes colorless diamond the king of gemstones

Diamonds themselves are colorless, but they will show a colorful effect under sunlight. This phenomenon is called fire. The fundamental reason for the fire is that the dispersion effect of diamonds is relatively strong. When white light passes through a prism made of glass, the prism will deflect the light. The ability of the prism to deflect different colors of light is different, causing the colors of light to scatter. This is the dispersion effect of light.

(Dispersion effect of light passing through a prism)

The dispersion value is used to evaluate the strength of the dispersion effect of a material. For materials with weak dispersion effects, the color is not dispersed much after the light passes through, and the dispersion value is low. Conversely, the dispersion value is high. The dispersion value of glass is about 0.009, while the dispersion value of diamond reaches 0.044. In comparison, the dispersion value of sapphire is only 0.018. Diamonds, which have no color themselves, with extremely high dispersion values, show amazing fire. With exquisite cutting and polishing technology, the fire can be more vividly reflected. Take ordinary transparent glass and cut it into the shape of a diamond to form a relatively good-looking fire.

(The brilliance of diamonds)

Strong dispersion ability is a good thing for gemstones, but it is an optical defect that needs to be corrected in eyeglass lenses or optical lenses. If the dispersion value of eyeglass lenses is too high, there will be colored edges on the edges of objects. Photos taken with lenses with dispersion will have purple edges on the edges of objects.

Cat's eye and starlight effect **, the light is **** smart**

Some gemstones will appear a bright band of light when exposed to light. As the light moves, the bright band will also move with it. This luster is similar to that of a cat's eye. This optical effect is called the cat's eye effect, and these gemstones are also called cat's eye stones.

Natural gemstones are formed in complex geological environments. The mixing of foreign impurities and changes in temperature and pressure will affect the growth of gemstones and leave certain traces inside the gemstones. These traces are called inclusions. There is a group of dense, parallel and oriented fibrous inclusions inside the cat's eye (like fine stripes distributed in parallel, with a size of about 0.01mm to 0.1mm). These inclusions refract and reflect light, producing the effect of bright light bands. For example, titanium dioxide (also known as rutile) impurities exist in cat's eye sapphires. Rutile is arranged in the sapphire in the form of fine fibers, which generate strip-shaped light spots under light.

The starlight effect is an upgraded version of the cat's eye effect. The internal inclusions of gemstones with starlight effect are cross-distributed fine fibers. Under light, multiple cat's eye stripes form starlight-like stripes.

This kind of effect is also very common in life. On rainy days, after the side windows of car glass are raised or lowered, fine water lines will form on the surface. The role of these lines is like the inclusions in cat's eye stones. When you look through the car window at night to observe the street lights outside, you will find that there is also a thin horizontal light spot accompanying the light source. Similarly, when the mobile phone lens is accidentally smeared with oil to form stripes in the same direction, the shooting light source will also produce a light spot perpendicular to the direction of the stripes (it can also be deliberately smeared with oil, you can try it). This is the scattering effect of light. The finer the structure, the larger the scattering angle: the horizontal structure of the vertical stripes is much finer than the vertical structure, so a large angle of scattering is generated in the horizontal direction, forming horizontal stripes. .

Play of color effect **, natural microstructures in gemstones**

There are nanometer (100-300nm) tiny structures inside gemstones. When light passes through these microstructures, interference and diffraction effects will occur, forming colorful spots.

The gemstone with the most obvious color change effect is opal, also known as opal. Opal is a hydrate of silicon dioxide (the main component of glass is also silicon dioxide). The figure below shows the molecular arrangement of opal. The silicon dioxide spheres are arranged regularly, and there are gaps in the middle of the spheres. These gaps are like natural microstructures, which can cause light to diffract. The color produced is related to the arrangement period and the wavelength of light. From a macroscopic perspective, it forms colorful light spots. Depending on the size of the gaps, the diffraction effect on light is also different, and ultimately it is manifested as opal with different colors, such as seven-color, five-color, and three-color opal.

Source: Reference 1

There are many diffraction phenomena in life. For example, DVD discs have circles of grooves on the surface. These grooves are about 500 nanometers wide and about 700 micrometers apart. Under sunlight, there will be colorful spots on the surface of the disc, which is caused by the diffraction effect of light. Butterfly wings are also covered with microstructures. The beautiful blue color of Morpho butterfly wings is also caused by the diffraction effect of light.

Artificial gemstones appear **, gemstones may no longer be scarce ? **

Everyone pursues beautiful things, but the rarity of gemstones makes people hesitate. Today, technological development has enabled humans to create the harsh conditions required for the birth of gemstones, such as artificial diamonds made by high temperature and high pressure methods. This method can imitate the formation process of natural diamonds by converting graphite into diamonds under extremely high temperatures and high pressures. In addition to being used in industry (diamonds are extremely hard and can be made into abrasives), the produced diamonds can also be processed into gemstones. Similarly, artificial rubies, sapphires, etc. are not only widely used in industry, but also allow everyone to buy beautiful decorations at a lower price.

Next time you admire gems, I will surely be even more impressed by the wonders of nature.

References:

[1] Zhang Beili. Systematic Gemology[M]. Geological Publishing House, 2006.

[2] Zeng Guangce, Zhu Yunhai, Ye Delong. Crystal optics and optical mineralogy[M]. China University of Geosciences Press, 2006.

[3] Yang Guoguang, Song Feijun. Advanced Physical Optics[M]. University of Science and Technology of China Press, 2008.

Author: Wu Qing, postgraduate student at Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences

Reviewer: Li Ming, researcher at the Institute of High Energy Physics, Chinese Academy of Sciences