I need help pricing a natural Alexandrite

This is the first time that I have had the opportunity to sell a natural Alexandrite and could really use some help. This gem is part of an estate collection that I am handling. Here is the gems information…

Natural stone- origin unknown

Cut: round brilliant cut by a master cutter

Transparency: Transparent

Color change: Great color change from a beautiful emerald green to a red purple

Carat: 1.69 cts

I need to know what the fair market value would be for a gem like this one. Thanks