I have been asked so many times what my favorite stone is. Magazine articles, interviews, clients etc. My stock answer is: all of them. Okay, today I made a decision. Tanzanite. Not so much the traditional, though I do love those, but rather the Fancy Zoisites. Tanzanite is the trade name given to the blue /purple stone that Tiffany & Company made famous shortly after its discovery in 1967. These gemstones are heat treated in the accepted manner to produce the striking and unique blue/violet color, which Tanzanite is famous for.
Fairly recently, the Tanzanite mines have gone deeper and extremely rare pieces of Zoisite were found. Pink, Orange, Green and Yellow crystals. 
These astonishing gemstones are not treated. Yummy colors that display the same amazing properties of pleochroism as the traditional Tanzanite. In other words, they radiate three different colors dependent upon the axis you are viewing. The green is mesmerizing. I envision swimming under water through veils of green/ violet blue colors. The pink reminds me of rainbow cotton candy, sort of soft and ethereal with hints of orange and green, while the yellow is crisp and bright with a violet and greenish tinge. Alas, I have not yet seen an orange in person, though not for lack of trying. It is definitely on my list!
Phew! I am so relieved to finally have an answer for that constant question. Definitely, my favorite gemstone is Tanzanite, particularly the Fancy Zoisite colors!
You know… I just saw a Russian Alexandrite that changes color from a rich bright green to a beautiful magenta red…
