Can "Healing Stone" be used to gain Stamina or Sanity beyond the investigator's initial values?
Your copy of Healing Stone may differ depending on what version of the game and/or which expansions you have. In particular, Dunwich Horror comes with a new version of this card. 
However, according to the rulebook (second printing), sanity/stamina as listed on the character's initial values are also their Maximum values. (emphasis mine)


  SANITY AND STAMINA An investigator begins the game with a number of
  Sanity and Stamina tokens equal to the values listed on the
  investigator’s sheet. These two values are the investigator’s maximum
  Sanity and maximum Stamina.
  While an investigator may gain and lose Sanity or Stamina over the course of the game, the investigator’s current Sanity or Stamina may
  never exceed the investigator’s maximum Sanity or Stamina.