The major sources listing values for gems do not yet catalog the blue-green variety so I have extrapolated from the data available on the other colors. Sinkankas lists fine blue stones of between .5 to 1.5 ct at $75 to $200 per carat. His estimate for blue Brazilian stones is $100/ct. The rarest of all varieties, a rich purple from Maine, tops the list at $250 per carat.
The degree of polish can vary on this soft stone due to skill levels of individual cutters, giving well polished stones premium value. As with most gems, saturation of color, size, and clarity are the major determiners of value.