The Sapphire (Greek: σάπφειρος sappheiros "blue stone") appears in two separate locations.
Inside the dark grounds of the Caverns, we find three Sapphires, which, in this particular case, are useless red herrings, as the Red Effigy guarding the bridge to Florentine's Observatory requests the offering of a Red Gem.
Secondly, the Sapphire constitutes one of the items that are necessary to solve the riddles surrounding the acquisition of the Key of Tears from inside Raysiel's Tower. Upon closer examination of this gem, Rebecca will comment on its slight dullness. In the game, the Sapphire is representative of Indigo, the colour between blue and violet.
This particular jewel can be picked from the exquisite stained glass window in the small Chapel in Raysiel's Tower. Along with six other gems, it has to be correctly placed according to the ROYGBIV colour spectrum on the star that has been etched on the floor. Completion of this riddle will yield us the Rainbow-coloured Key which is used for opening the door inside the Nightingale Floor Room, allowing us to retrieve the Coloured Gem from a small casket, required to access the shimmering walkway in the main hall of the Room of Riddles.
Adam: What would you call this Rebecca?