When it comes to a color-change Gemstone there is more to learn than Color-Cut-Clarity-Carats, or the 4-C’s as they are commonly known. Gemstones that change color depending on the light they are seen in (also called Phenomenal Gems), are judged by different standards than other gems.
Learn about the history and beauty of these most desirable of gems and how the experts grade them. Learn how to buy Alexandrite on the Net or at your neigborhood jewelry store.
Color Changing Gems And Chemical Composition Of Alexandrite
In the world of gemstones, there are those special and rare few gems that exhibit a mysterious change of color dependent on the light they are shown in. These gems are called color-change or Phenomenal gems. Although color-change tanzanites, sapphires, quartz, spinels, crysoberyls, garnets, tourmalines, and fluorite (to name a few) occasionally can be found there is only one known gem that always has a color change: Alexandrite.
Alexandrite is the only gem that has a standard and that standard states the gem must change from green in daylight to purple or reddish purple (the more red the better) in incandescent light or candlelight.
Color-change in other gems run a gamut of colors: from purple to pink in Amethyst, purple to blue or even green in Tanzanite, green to red or yellow in Garnet, green to yellow or orange or blue in Tourmaline; the changes and colors run the spectrum of the rainbow. Alexandrite is the only gem that has a standard and states the gem must change from green in daylight to rich ruby red or violet/purple in incandescent light or candlelight.
Chemical Composition Of Alexandrite And Color-Change Chrysoberyl
Alexandrite is a specific type of color change chrysoberyl. The chemical composition of chrysoberyl and color change chrysoberyl is BeAl2O4. Chrysoberyl is in the Class called Oxides. The crystal system is orthombic and the crystals themself are commonly twinned which results in a triangular shaped crystal or a crystal within a crystal. While I could list all the known properties of chrysobery the most important issue is that Natural Alexandrite, which is chrysoberyl has at least one important difference between itself, normal chrysoberyl and color change crysoberyl. In Natural Alexandrite there is a replacement of some of the alumina in the molecular structure by chromic oxide causing Natural Alexandrite’s specific coloration. Color change chrysoberyl does NOT have this replacement.
The color of a GIA Type 2 gemstone is of paramount importance. The more pure the color (hue), the less a difference cut, clarity and carat weight have on the beauty and value of the stone.
Color is subjective; however in colored gemstones there is a standard that is considered the appropriate and most beautiful color for that gem. In color-change gemstones, of which Alexandrite is the only one in nature in which 100% of the stones change color; the Standard states the gem must show as bluish green or Emerald green in daylight or fluorescent light and red or purple in incandescent or candlelight.
The color and strength (percentage) of color-change is another important factor. Most gemologists refer to the strength of the color-change by percentage (with 100% being ideal and stones below 30% not considered) or terminology (weak, moderate, strong). Alexandrite with 100% color-change will show completed color-change on every axis of the gem– all the facets will change color. Looking down through the table of the gem, if half the facets change color the gem is classified as a 50% change, if three-quarters change color it is a 75% color-change etc.
Color is described in gems through: Hue– the exact color in the spectrum (red, orange, yellow, green, blue, indigo, violet); Intensity or Saturation– the brightness or dullness of the color; Tone– how light or dark the stone is (how much black or white is present); Distribution– the evenness of color throughout the stone (Zoning). When reading a description of the color of a gem if there are mixed hues, the predominant hue is Capitalized and the lesser hue is shown in lower case. For example: bluish-Green; Green is the predominant color and blue is the lesser (underlying) color.
Note that in the last few years, unknowing or unscrupulous sellers are marketing color-change chrysoberyl as Alexandrite. Although Alexandrite is a specific form of chrysoberyl with both having a chemical composition of BeAl2O4, it is the very small replacement of alumina in chrysoberyl by chromic oxide that causes the alexandrite’s characteristic and standard coloration.
Gems have been standardized in weight since 1913. All gems (except pearls and coral) are weighed in Carats. One carat equals 200 milligrams (1/5 of a gram). The Carat is a unit of weight- not size! A rule of thumb: multiply the gram weight of a stone by 5 and you will have the carat weight (example: 0.06 grams=0.30 carats).
Gems have different densities. If you take a one carat ruby and compare it to a one carat emerald there will be a difference in size because ruby is denser than emerald. Therefore, the ruby will be smaller than the emerald even though both WEIGH the same and therefore have the same carat weight.
For each type of gemstone- sizes (in millimeters) will yield specific weights (carats). To reference Natural Alexandrite, for example: round stones of determined mm (millimeters) will always fall into a very small range of carat weights such as a 2mm= .05 carats, 2.5mm= .08 or .09 carats. Any large deviation from the referenced sizes would mean that the stone was either cut shallow (example: 2.5mm= 0.4 carats) or deep (example: 2.5mm= .14 carats). If a stone is cut too shallow the color will be washed out or too light, and if a stone is cut too deep the stone will appear too dark).
Mining of gem quality alexandrite is extremely rare in sizes over .25 carat. Stones that are less than .50 carats can sell for thousands of dollars. Stones over one carat usually cost in the tens of thousands of dollars for fine quality, while stones over three carats, that are naturally mined, can cost close to $100,000.
Clarity can be described as the degree of absence of inclusions (foreign particles, bubbles, fissures etc. in a stone). Alexandrite is a Type 2 GIA gemstone, which means that the norm is for a stone to have minor inclusion (silk, negative crystals, fissures etc). A clean faceted Alexandrite up to a carat in size is rare and above one carat is extremely rare. Therefore, a VVS Alexandrite (which is the highest classification) is described as: Very Very Slightly Included – Minor inclusions.
Alexandrite is found in several shapes with oval facet being the most common (and having the least waste material after cutting). Round has the most waste after cutting which renders the stones a premium in price. More alexandrites are now being cut in other shapes such as, pear, cushion, emerald, fan, baguette and fancy cuts.
Most alexandrite are native cut- that is, they are cut and faceted in the country in which they are mined by craftsmen that have been trained from father to son over many generations. The cutting and faceting is all done by hand and not by machine.
Natural Alexandrite Prices of all origins as of May 1, 2008 as reported by the International Gem Society are as follows:
Top Red/Green (90%-100% Color Change)
$2,500 to $6,000/ct+
$5,000 to $15,000/ct+
1 carat and up
up to $1,000,000/ct
Medium Red/Green (70%-89% Color Change)
$1,500 to $6,000/ct
$1,500 to $9,000/ct
1 carat and up
up to $60,000/ct
Slight Red/Green (30%-69% Color Change)
$100 to $2,500/ct
1 carat and up
up to $6,000/ct
Cabochons – Strong Red/Green
$500 to $2,500/ct
1.5 carat and up
up to $30,000/ct
Cat’s Eye – Strong Red/Green
$1,500 to $5,000/ct
Top Red/Green does not mean that the alexexandrite changes from Ruby Red to Green. It is well known that 99.99% of all alexandrite do not exhibit such a color change. Rather, the well known coloration is Purple/Green. The more Red in the gem in warm incandescent lighting the more valuable the stone.
Other Coloration in Alexandrite: Any color change that is not Green/Purple. The daylight coloration must be a shade of green, and the incandescent coloration a varient of red/purple such as rust, brownish, orangey.
Retail price is determined not only by the value of the gemstone or jewelry itself but also by market strength, geographical location, and other factors including, but not limited to; cut, proportioning, color-change, finish.
True synthetic Alexandrite have been lab grown in Russia from Corundum for the past ten to twelve years. These stones are being marketed in some quarters as Russian Alexandrite, Natural Alexandrite, and even Natural Russian Alexandrite. As early as 1907 or 1908 synthetic Alexandrite were created from synthetic color-change sapphires colored by vanadium. Since these stones have been in circulation for several generations it is entirely possible for families to think that an heirloom Alexandrite is a true natural, mined, Alexandrite. Visitors to foreign countries buy heirloom Alexandrite thinking it is natural when, in fact, many times it is a cheap synthetic sapphire worth only a few dollars.
Hopefully this article has answered a few of your questions and you now have a better undestanding of what is important and how to buy Alexandrite.