Source: Parshat Mishpatim – משפטים

They saw a vision of the
God of Israel and under His feet
something like a sapphire brick
like the essence of a clear blue sky.
לִבְנַת הַסַּפִּיר וּכְעֶצֶם הַשָּׁמַיִם לָטֹהַר

G-d’s “bricklaying” was in empathy
with suffering of the Jews in Egypt
who were forced to build the pyramids,