Alexandrite

Alexandrite's celebrated colour change, often summarised as emerald by day, ruby by night, arises because chromium in the chrysoberyl structure transmits both green and red light almost equally, letting the light source decide which dominates. Daylight and fluorescent light bring out bluish green to green; incandescent bulbs and candlelight reveal purplish red to raspberry hues. The strength and quality of the change vary enormously between stones: the finest show a distinct, dramatic swing between attractive colours, while lesser stones shift weakly between greyish or brownish hues. Sri Lankan alexandrites have historically included larger crystals than many sources, often with olive-toned greens. The colour change, not just the colours themselves, defines this gem.

The dominant quality factor is the colour change: its completeness, the attractiveness of both colours and the absence of grey or brown. Laboratories and dealers often describe the percentage of colour change; a strong shift between pleasant green and purplish red marks a fine stone. Clarity comes next, eye-clean alexandrite is rare, and transparency should be good enough to display the colours vividly. Cut quality affects how convincingly each colour presents. Size matters greatly: fine alexandrite over a carat or two is exceptionally scarce, and prices escalate steeply. Certification from a recognised laboratory is essential, both to confirm natural origin and to distinguish alexandrite from the abundant colour-change synthetics and simulants.

Alexandrite is typically untreated, no routine enhancement improves it, and its value rests on natural colour change. Any treatment that did occur would have to be disclosed, as with every gem, but the practical marketplace issue is different: synthetic alexandrite and colour-change simulants are widespread. Synthetic colour-change sapphire and synthetic colour-change spinel have been sold as 'alexandrite' for a century, especially in tourist markets and estate jewellery, and true synthetic alexandrite is also produced. A recognised gemmological laboratory can separate natural alexandrite from all of these. Given the gem's price level, no significant alexandrite should be purchased without a current laboratory report confirming natural origin.

Alexandrite was first recognised in the emerald mines of Russia's Ural Mountains in the 1830s and named in honour of the future Tsar Alexander II. As Russian production declined, other sources emerged: Sri Lanka became an important supplier, and today Brazil, India, East Africa and Madagascar also produce the gem. Sri Lankan stones are recovered from the island's gem gravels, sharing localities with chrysoberyl and cat's eye around districts such as Ratnapura and Elahera. Origin can influence value, with fine Russian and Brazilian stones especially celebrated, but the individual stone's colour change ultimately matters more than its passport.

Alexandrite is an excellent wearing stone: hardness 8.5, exceeded only by corundum and diamond among common gems, with good toughness and no problematic cleavage. Clean with warm soapy water and a soft brush; ultrasonic and steam cleaners are generally safe given the gem's typically untreated state, though caution is sensible with included stones. Protect it from hard knocks as with any gem, and store it separately from diamonds and corundum that could scratch it. Given alexandrite's value, professional inspection of settings once or twice a year is a worthwhile habit, and insurance appraisals should be kept current with laboratory documentation.