Eight cut is also known as single cut. In case of eight cut there will be eight crown facets, eight pavilion facets and one table facet i.e. all together 17 facets. Eight cut is generally used in those gemstone where round brilliant cut or other cuts are not possible.

If the stone is completely opaque then your stone could be either chrysocolla or turquoise on the basis of color teal (greenish-blue). But it would make sense if you can post the picture of the gemstone.