An apparatus for selectably isolating a subterranean formation from a production string, the apparatus comprising: an elongate outer casing having an interior thereof extending therethrough, said outer casing having a plurality of apertures an exterior wall thereof; an upper sleeve extending between first and second ends (44, 46) slidably located within said interior of said outer casing forming an annular void therebetween, said upper sleeve forming a central passage therethrough; an annular piston slidably displaceable within said void between said outer casing and said upper sleeve so as to be positionable to selectably obstruct said plurality of apertures through said outer casing; a lower sleeve located within said interior of said outer casing below said upper sleeve biased towards said upper sleeve, said lower sleeve forming a central passage therethrough in common with said upper sleeve; characterized in that, the apparatus further comprises an expandable ring located between said upper and lower sleeves (40, 42); a backing body located within said interior of said outer casing and having a retaining portion adapted to bias said expandable ring to radially extend inwardly into said central passage, said backing body having a recess therein adapted to permit said expandable ring to expand radially outward when displaced by a predetermined distance by a first spherical object upon said expandable ring, wherein said expandable ring selectably retains said spherical body thereon when biased inwardly by said retaining portion of said backing body so as to be operable to pressurize said central passage thereabove such that said pressure displaces said annular piston so as to uncover the apertures. an elongate outer casing having an interior thereof extending therethrough, said outer casing having a plurality of apertures an exterior wall thereof; an upper sleeve extending between first and second ends (44, 46) slidably located within said interior of said outer casing forming an annular void therebetween, said upper sleeve forming a central passage therethrough; an annular piston slidably displaceable within said void between said outer casing and said upper sleeve so as to be positionable to selectably obstruct said plurality of apertures through said outer casing; a lower sleeve located within said interior of said outer casing below said upper sleeve biased towards said upper sleeve, said lower sleeve forming a central passage therethrough in common with said upper sleeve; characterized in that, the apparatus further comprises an expandable ring located between said upper and lower sleeves (40, 42); a backing body located within said interior of said outer casing and having a retaining portion adapted to bias said expandable ring to radially extend inwardly into said central passage, said backing body having a recess therein adapted to permit said expandable ring to expand radially outward when displaced by a predetermined distance by a first spherical object upon said expandable ring, wherein said expandable ring selectably retains said spherical body thereon when biased inwardly by said retaining portion of said backing body so as to be operable to pressurize said central passage thereabove such that said pressure displaces said annular piston so as to uncover the apertures.