This is one of the "hand held size" specimen I used to produce in quantity several years ago.
As the nickel is only contained in traces you get a more blueish color instead of the expected green.
The nickel is realy sucked out of the solution during the growing process remaining a practical colorless solution
with a very low nickel concentration. The nickel concentration in the crystal is several times higher as in the
corresponding solution.
Higher concentrations of nickel will result in "greener" shades with the pure ammonium nickel
sulfate having a slightly blueish dark green color which resembles a lot to dioptase.
Funny enough as it happens often with dioptase too, if you take a photograph it will mostly appear to blue on the picture.
This specimen is currently exhibited at the mineral museum in Pforzheim
