Lab Vs Natural Alexandrite: What’s the difference?
Before I go into the differences, let's just sum up a few details. Alexandrite is a form of a mineral known as chrysoberyl and the color change is a completely natural phenomenon.
Emerald by day, ruby by night.
Alexandrite is a pleochroic gem that displays a beautiful green in daylight and transitions to smokey purple brown in candlelight.
Pleochroics are gems that appear to have different colors or depth of color when viewed from different angles. Most pleochroic gems show hints of different colors in other light, but alexandrite changes color completely. The main pleochroic colors are: orange, green, and purple-red-its the way the gem reflects back light.
Red to green color change is caused by strong absorption of yellow light which allows more red and green to be displayed. The hue depends on the angle of illumination. Incandescent light causes a reddish hue while daylight brings out this blueish green.
How is it formed?
The chrysoberyl forms in pegmatite rocks, which are a result magma rising to the surface of the earth and cooling. The magma becomes rich not only in water but rare elements that couldn’t be incorporated in the crystallization of solid minerals. The water helps lower the temperature of the magma before it solidifies which allows it to develop its own minerals. The result: This magmatic rock filled with huge crystals of minerals such as beryllium (the key composition of alexandrite).
This alexandrite effect is the result of a small amount of aluminum replaced by chromium. Because our eyes are most sensitive to green light and least to red, we see green in the daylight and red color at night.
Now let's get into the creation process of synthetic alexandrite.
Synthetic alexandrite is an artificially grown type of chrysoberyl crystal called Beryllium Aluminum oxide.
There are two most common methods for creating synthetic alexandrite: Czoralski process and the “floating zone.”
GIA
In 1916, Jan Czochralski invented a crystal pulling method to synthesize rubies and it will later become the same method of producing synthetic alexandrite.
Czochraslki Process
Seed crystal is attached to a rod and is lowered into a crucible until the crystal touches the melt (liquied solution to grow the gem). After the rod is taken out, the crystal grows as the material solidifies. Crystals grown this way can grow to more than one meter in diameter and 50 meters in length with a high level of purity. Alexandrite created using this process is known as alexite.
Floating Zone Method
Floating zone method was developed by an Armenian scientist named Khatchatur Saakovich Bagdasarov of the Institute of Crystallography of the Russian Academy of Moscow in 1964. The jewelry market picked it up years later.
The floating process is quite similar to the Czochralski method except it uses a tungsten dish pulled over a period of time under a heating element. The time depends on the size of the dish and the material being crystallized.
Alexandrite made by floating method tends to lack in luster compared to the Czochralski method. Think of a cotton candy machine.
The flux method has also been another option to produce alexandrite but this process can contain remnants of undissolved flux which will appear as an inclusion-this is also an easy way to tell the difference between natural and synthetic. This is why the flux process is no longer used.
Photo courtesy of GIA
Lab created and natural alexandrite start with the same chemical makeup.
Scientists know how natural alexandrite is formed and the conditions so they try to imitate as closely as possible, resulting in the same physical and chemical characteristics.
Alexandrite rates an 8.5 on Moh’ scale which makes it perfect and quite practical for everyday wear. You can wear it without fear of it chipping and/or cracking.
Clarity
Natural alexandrite is considered a Type II clarity stone, which means they are typically eye clean. They may show some inclusions such as tiny crystals, silky threads, throughout the whole or portion of the gem or have fissures that make it look like the gem appear to have a crack.
The Czochralski method creates alexandrite that is free of all inclusions. However, flawless gems are not easily found in nature, so it is easy to spot a synthetic alexandrite. Although this process is pricey, the cost of synthetic gem is a small compared to the cost of natural alternative.
Lab and natural with both display the pleographic properties.
There are a few imitations of real alexandrite. Color change sapphire has been passed off as alexandrite for years.
In order to identify a real alexandrite, it takes knowledge and experience. Therese has both and all the tools to test the gem!
Most gemstones that claim to be alexandrite are actually corundrum (also known as sapphire) laced with vanadium to produce color changing properties. Although still pleasing to the eye, they don’t exhibit the green color that produces the emerald by day effect.
Sapphires tend to be clean of inclusions and are available in suspiciously larger sizes.
Alexandrite is typically only found in smaller sizes, often less than one carat. Gemological testing will reveal a slightly higher refractive index, a key indicator of a gem’s identity.
Sometimes alexandrite simulants are made of glass, specifically doped glass. The brilliance is nowhere near that of true alexandrite.
Let Therese help you with your alexandrite needs! See my original post on alexandrite for more information on the natural alexandrite!