Tablets of Moses

Chagall 

The "sapphire" of the Bible is really Lapis Lazuli:

"According to the biblical narrative the first set of tablets, inscribed by the finger of God, (Exodus 31:18) were smashed by Moses when he was enraged by the sight of the Children of Israel worshipping a golden calf (Exodus 32:19) and the second were later chiseled out by Moses and rewritten by God (Exodus 34:1). Exodus 34:28 says the second set were written by Moses.

"According to traditional teachings of Judaism in the Talmud, they were made of blue sapphire stone as a symbolic reminder of the sky, the heavens, and ultimately of God's throne. Many Torah scholars, however, have opined that the biblical sapir was, in fact, lapis lazuli (see Exodus 24:10, lapis lazuli is a possible alternate rendering of "sapphire" the stone pavement under God's feet when the intention to craft the tablets of the covenant is disclosed Exodus 24:12)."

https://en.wikipedia.org/wiki/Tablets_of_Stone

The Tablets of the Law were fittingly made of Lapis Lazuli:

“Two things fill the mind with ever new and increasing admiration and awe, the more often and steadily we reflect upon them: the starry heavens above me and the moral law within me."

-Kant

The most conspicuous property of lapis lazuli indeed is the heavenly blue color with the golden specks of very small pyrite crystals, often compared with the blue sky and golden stars.