Al-Quran is described by God Almighty as hidden within a protective shell (مكنون) similar to that which surround eggs or pearls as per by the following verses:
37:49  كأنهن بيض مكنون   as if eggs hidden inside protective shells
52:24  كأنهم لؤلؤ مكنون  as if pearls hidden inside protective shells
56:23  كأمثل ٱللؤلؤ ٱلمكنون  As if pearls hidden inside protective shells
56:78  في كتب مكنون  In a book that is hidden inside a protective shell
56:79  لا يمسه الا المطهرون  No one reaches it except the purified in [33:33]

