what is juana and kinos reaction to the oyster

This actually happens in Chapter 2. Both Kino and Juana are ecstatic although not for entirely the same reason. The "ghostly gleam" of the shell seems to have improved the wound on their baby. Juana is keenly aware of this first. Kino is just excited about how big the pearl is and how the money might change their lives. Steinbeck places this Pearl at a desperate moment in their lives so their reactions are that much more important to how the novella unfolds.