Alexandrite
What colour gem would you like? Ruby red? Emerald green? Or purple like amethyst? Why not buy alexandrite and have them all!
The most striking feature of alexandrite is its colour changing properties. Under natural sunlight it's emerald green in colour but under some forms of articial light it changes colour completely to be come red, purple or violet. This strange property is so closely associated with alexandrite that the word alexandritic is applied to any colour-changing gemstone.
Pirates of old would probably have used a simpler word, perhaps along the lines of "magical". Alexandrite was unknown then and was only discovered in the first half of the 19th century. If it had had been known in the classic pirate era then it would undoubtedly have been greatly valued as booty; genuine natural alexandrite is extremely rare and valuable. Unless you're prepared to pay premium prices most of the alexandrite jewelry you can buy today uses artifical lab-created stones.
On a more prosaic level, artifical alexandrite crystals are used in lasers for removing tattoos and unwanted hair - something that would have been of great use to many pirates!