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ALEXANDRITE

 

                    

Introduction:  Alexandrite is a variety of the mineral species chrysoberyl.  Pictured is a 5.05 ct alexandrite from Sri Lanka shown in daylight on the left and incandescent on the right.

Colors:  Alexandrite is a well known color changing material as the light changes.  In daylight, colors are yellowish-green, brownish-green or bluish-green.  In incandescent lights, colors are orangy-red, brownish-red, reddish-purple or at best purplish-red.  Many gemological texts show photos of the color change from an emerald green to a stop light red.  Although this would be an ideal color change and is the color change that is sought after, it rarely if ever exists.  The quality of the color change is the primary basis to determine the value of an alexandrite.

Clarity:  Alexandrite is of Type II clarity.  Gems in this type typically grow with some minor inclusions in nature that may be eye visible but usually are well hidden or require magnification.  Typical internal characteristics of alexandrite include fingerprint inclusions, mica, calcite, pyrite and silky fibers of sillimanite responsible for cat’s eyes.

Stone Sizes:  The largest alexandrites from the classic Russian locality or Sri Lanka are up to 50 carats.  Alexandrites that exceed 5 carats in size from any locality would be considered large.

Localities:  The classic source for alexandrite is Russia, near Sverdlosk.  The color change is typically a reddish-purple to purplish-red in incandescent to a green in daylight that has a hint of blue as a secondary color.  It is also found in Brazil usually with a mauve incandescent color and a strong bluish component to the green daylight color.  Sri Lankan stones tend to have a strong brown overtone on the incandescent color and an olive-green daylight color.  Alexandrites from Zimbabwe have an exceptionally good color change with a strong emerald green daylight color but fine stones are rarely over 1 carat.  Tanzania, Madagascar and India are additional sources for alexandrite.

Treatments:  None

Gemology:
-Refractive Index:  1.746-1.755 (+0.004,-0.006)

-Birefringence:  0.008-0.010

-Optic Character:  Doubly refractive, biaxial positive

-Dispersion:  0.015

-Specific Gravity:  3.73 (+/- 0.02)

-Hardness:  8.5

-Toughness:  excellent to good

-Chemical Composition:  BeAl2O4

-Cause of Color:  Chromium

-Absorption Spectra:  two strong lines at 680.5 and 678.5 nm; weak lines at 665, 655 and 645 nm; partial absorption between 580 and 630 nm; three weak lines at 476.5, 473 and 468 nm; general absorption in the violet

-Fluorescence:  inert to moderate red (LW and SW)

-Cleavage:  indistinct

-Phenomena:  Chatoyancy

Name:  Alexandrite is named after Czar Alexander II of Russia, on whose birthday the gem was found.

Dates:  Alexandrite is an alternate birthstone for June and the 55th anniversary gemstone.

Care:  The ultrasonic and steam cleaner are usually safe.  Warm, soapy water is always safe.


To see available alexandrites, click here.