Can these white stones “replicate” the legendary “invisibility technique”?

If you could become invisible, what would you do?

There was once a real and feasible invisibility technique circulating on the Internet in an emergency: take natural calcite, it must be natural, because it has absorbed the spiritual energy of heaven and earth for thousands of years. Choose a Ding day to bathe and change clothes, burn the calcite to white powder, sieve it with a fine mesh, keep it dry and store it for later use. In an emergency when being chased, run against the wind to absorb the spiritual energy of heaven and earth, then turn to face the pursuer, and silently say "Hurry up as the law orders, I will be invisible!" When the pursuer approaches, sprinkle the white powder on his face to instantly become invisible. If you are (touch) enough (have) enough (have) heart (violent) sincere (strike), you can also achieve permanent invisibility. I believe you have already laughed a little after seeing this. This is not an invisibility technique, it is clearly Wei Xiaobao's unique killer move - spreading lime powder. Laughing aside, this behavior should not be done even if it is unethical, but calcite is indeed related to invisible materials.

In The Deer and the Cauldron, Wei Xiaobao attacked with lime powder and was reprimanded (the picture comes from the Internet, if it infringes, please delete)

In 2011, scientists from the United States and Singapore jointly developed a low-cost "invisibility cloak" made of transparent calcite crystals, which can achieve invisibility under visible light conditions at specific angles. Its principle is to use the optical property of transparent calcite crystals - double refraction.

Before I go on, let's talk briefly about calcite. Calcite is a very common mineral, especially in areas with developed karst landforms, where it can be found everywhere. It is composed of calcium carbonate, and it is named because it has complete cleavage and can break into small cubes when dropped on the ground. In Yunnan, people think that the small cubes that are broken look like horse teeth, so they call them horse teeth stones. After calcining calcite at high temperature, quicklime (calcium oxide) can be generated. The "Lime Chant" written by Yu Qian, a politician in the Ming Dynasty, actually reflects this process from a scientific perspective.

Ming Dynasty - Yu Qian - "Lime Chant" (Picture from the Internet, please delete if infringed)

Calcite crystal specimens in the collection

Common calcite is white. When it contains coloring elements such as iron and copper, it can be yellow, red, blue and other colors. There are also colorless and transparent varieties. This kind of calcite was first discovered in Iceland, so it is also called Iceland spar. It has the largest double refraction among known minerals. Double refraction is the phenomenon that one incident light produces two refracted light rays. The simplest and most direct way to observe the double refraction phenomenon of Iceland spar is to put it flat on a piece of paper with words written on it. When you look through it, you will see that the words have double images.

Double refraction of Iceland spar (picture from the Internet, please delete if infringed)

The application of the double refraction phenomenon of Iceland spar can be traced back to the Viking period. In the legend of the Viking pirates, there is a mineral called sunstone that can help them identify the position of the sun on cloudy days. Because whether it is cloudy or sunny, there will be a group of concentric rings in the sky with the sun as the center and composed of different polarized light. When the light from the sky passes through the mineral crystal, it will be divided into two beams and project images with different brightness. At this time, the direction of the crystal is slowly changed until the brightness is adjusted to be completely consistent. With a little bit of navigation experience, the position of the concentric rings can be known and the position of the sun can be determined. The so-called "sunstone" is Iceland spar.

Using the double refraction of Iceland spar to locate the sun (Image from the Internet, deleted if infringed)

Nowadays, we can rely on technology to achieve more accurate and convenient positioning at sea, and the stealth materials mentioned above have not yet reached the stage of practical use. However, with its advantages such as large birefringence, stable properties and low cost, Iceland spar still plays a big role in the modern optical industry and can be used in many fields, such as macrometers in the astronomical field, polarization beam splitters in optical communications, and dichroic mirrors for gem identification.

Diagram of the structure of a dichroic mirror for identifying gemstones (picture from the Internet, please delete if infringed)

The pleochroism of ruby ​​under a dichroic microscope, with different shades of red on the left and right (picture from the Internet, please delete if infringed)