"And the tablets were the work of God" (Exodus 32:16). And it says, "And they saw the God of Israel, and under His feet was like a work of sapphire stone" (Exodus 24:10). This compares one "work" to the other "work": just as the "work" mentioned there was of sapphire, so too the "work" mentioned here was of sapphire.
The Tablets Were Sapphire Like the Pavement Beneath God's Feet
Yalkut Shimoni on Torah 392:9
וְהַלֻּחֹת מַעֲשֵׂה אֱלֹהִים הֵמָּה. וְאוֹמֵר "וַיִּרְאוּ אֵת אֱלֹהֵי יִשְׂרָאֵל". מַקִּישׁ מַעֲשֶׂה לְמַעֲשֶׂה, מַה מַּעֲשֶׂה הָאָמוּר לְהַלָּן, שֶׁל סַפִּיר, אַף מַעֲשֶׂה הָאָמוּר כָּאן, שֶׁל סַפִּיר.