An apparatus for selectably isolating a subterranean formation from a production string comprises a radially expandable ring located between upper and lower sleeves and retained at a position to intrude into a central passage extending therethrough. The expandable ring is permitted to radially expand into a recess upon displacement of the lower sleeve to permit a body retained thereon to move therepast. The apparatus includes a pair of inner and outer indexing rings which cooperate to permit the inner and outer indexing rings to move towards a bottom end thereof only to a fixed endwall whereafter further movement of the inner and outer indexing rings and lower sleeve is prevented.