Alexandrite - a gem for a gem

This is an example of Alexandrite, a very rare mineral first discovered in the Ural Mountains of Russia in 1833 and named after the future Tzar Alexander II when he turned 16 a year later.

It has the very unusual property of appearing green in daylight and red in incandescent light, making it even more fortuitous discovery as these were the principal colours of Imperial Russia. It became very popular and some examples of Alexandrite jewellery survive to this day but are extremely rare and valuable. It proved to be very difficult to photograph, not least because this piece is very tiny, only a couple of mm across. I think you can see the red and green a little bit though.

It is a form of Chrysoberyl which contains chromium as an impurity which is what causes its interesting optical properties. According to my mineral book it will block solar radiation as well but I can't find any reference to this online.

Green and Red also feature very strongly on the Welsh flag which is another reason why Alexandrite makes a smashing tribute to the fabulous Ceridwen who posted her 500th blip yesterday. :-)

Here is a website which knows everything there is to know about Alexandrite.