No, this is not an aquamarine, but a gemmy, tabular APOPHYLLITE crystal, 3 cm across! It sits up on its edge right next to two accenting stilbites. The crystal is actually clearer than it looks: you are seeing a knob of matrix right through the crystal itself. A quite uncommon specimen of a too-common species, and beautiful at that. 7 x 6.3 x 4.5 cm