Lapis Lazuli - Improving color

HELP ME!

I have got 10 tons lapis lazuli from Afganistan. I have al ready
done 50.000 pieces cabochon. Half of them have light colour. How to
improwe de colour. Who knows the methods and technologe? Thank you
in adwance. Best regards. Janusz Mazurek, Warsaw, Poland tel/fax
+48 22 6286275 @Janusz_Mazurek