Rashi on Exodus 24:10/רש"י על שמות כ״ד:י׳

1ויראו את אלהי ישראל NOW THEY SAW THE GOD OF ISRAEL — They gazed intently and failing in this they peeped in their attempt to catch a glimpse of the Supreme Being, and thereby made themselves liable to death. But it was only because God did not wish to disturb the joy caused by the Giving of the Torah, that He did not punish them instantly, but waited (postponed the punishment) for Nadab and Abihu until the day when the Tabernacle was dedicated, when they were stricken with death, and for the elders until the event of which the text relates, (Numbers 11:16) “And when the people complained …. and the fire of the Lord burned among them and destroyed בקצה המחנה” — those who were the קצינים “nobles” of the camp (Midrash Tanchuma, Beha'alotcha 16).

2כמעשה לבנת הספיר AS IT WERE THE BRICKWORK OF SAPPHIRE — This had been before Him during the period of Egyptian slavery as a symbol of Israel’s woes — for they were subjected to do brick-work (cf. Jerusalem Talmud Succah 6:3; Leviticus Rabbah 23:8).

3וכעצם השמים לטהר AND AS IT WERE AS THE BODY OF HEAVEN FOR PURITY — This implies that as soon as they (the Israelites) were delivered there was radiance and rejoicing before Him.

4וכעצם — Translate it as the Targum does: “as the appearance”.

5לטהר means FOR BRIGHTNESS AND CLEARNESS.