Purchasing Alexandrite over the internet can be simple. The seller should disclose the Origin of the stone, the Color and Strength of color-change, Cut, Size, Weight, Clarity, any Treatment and if the stone is a Natural, not synthetic or lab created stone. Look for size as it relates to carat weight. For example:
2.0mm = .05 carat +/-
2.5mm = .08 – .09 carat
3.0mm = .15 carat +/-
3mm X 2mm= .10 carat +/-
4mm X 3mm= .35 carat +/-
5mm X 4mm= .50 carat +/-
Check out the Clarity. Know that larger stones (over one carat) almost always have small inclusions and stones over 2.5 carats always have inclusions (unless they are over $100,000 and certified). Heirlooms most possibly are synthetic: look out for large carat weight with few, if any inclusions.
Check out color-change. Check to see that the pictures shown are taken in natural light and not filtered or photoshopped. If the pictures are taken in ideal lighting conditions with special light boxes etc, chances are most likely the gem will not appear the same color once it is seen in natural conditions. Also, note that different types of monitors (flat screen, CRT etc.) and different brands of monitors will show colors slightly differently and that settings are set subjectively (such as brightness and contrast). Therefore, carefully read the written description of the color and color-change.
See if there is Certification by an independent lab or a Certificate of Authenticity offered by the seller. Several countries, such as India, have Governmental Regulatory Agencies that will certify the gems.
Know the seller and the seller’s reputation. Call or email the seller to see if questions are answered knowledgebly and promptly. Read testimonials. Check that the seller is a member of professional organizations in the gem and/jewelry industry.