An inclusion is a sapphire with a pocket of air trapped in the stone.
In the short story "The Witcher", Geralt explains to King Foltest some precautions to take to maximize the chances of Princess Adda's recovery from the curse:
“
‘This is a serious matter, your Majesty. The risk is great. That is why you must listen: the princess should always wear a sapphire around her neck, or better, an inclusion, on a silver chain. Day and night.’
”
In The Witcher computer game[ | ]
In the original game, there was a quest item which was a "A sapphire with an air bubble inside" (ID number it_quest_155) which did not make the final cut.