but the most likely scenario is that "silky aquamarine" is actually what is know as black beryl. Black beryl gets its color from micro inclusions of black spinel which oriented along the plains perpendicular to the optical axis. Black colour is observed only in the center of such crystals. The surface of the crystals, can be few inches deep, is grayish-blue and if cut would produce something which can be called "silky aquamarine". Strictly speaking, it should never be called and aquamarine.
While I’m deeply respectful of Mr. Surpin’s opinions, which are
generally right on the mark, I’ve cut some material like that, and it
certainly didn’t appear to be included with anything black. The
"silky" inclusions permeated the crystal toward the base - it was
clear at the tip, and it wasn’t any blacker in the center than on the
outside. The whole thing was the light-blue color typical of
aquamarines, without a grayish hue. Personally, I thought it made a
more interesting cabochon than the clear material, since it exhibited
chatoyance in strong light. The fibers looked a lot like the "silk"
found commonly in star sapphires, which I believe is caused by
rutile, not spinel. I’m not sure if it’s more or less valuable than
equivalantly-colored clear material, but since aquamarine is pretty
common stuff anyway, I’m not sure the difference was significant. The
customer who got it in a custom brooch seemed to like it a lot…