O thou afflicted, tossed with tempest, and not comforted, behold, I will lay thy stones with fair colours, and lay thy foundations with sapphires.
And I will make thy windows of agates, and thy gates of carbuncles, and all thy borders of pleasant stones.(Isaiah 54:11-12) KJV
The firmament was originally composed of many more layers than it is now. One layer in the midst transformed into something like obsidian, completely black, and melted causing the inner layers to separate.
The layers underneath it sloughed off, broke part, and became what are known as the fixed stars. The remaining firmament which is still in place is constructed of a series of different crystal forms, including sapphire.