Speak to the Israelite people and instruct them to make for themselves fringes on the corners of their garments throughout the ages; let them attach a cord of blue to the fringe at each corner.
פתיל תכלת. על הציצית. וטעם תכלת. בעבור שהוא כעין השמים:
It is because sky-blue dye is similar in its color to the sea, and the sea is similar to the sky, and the sky is similar to the Throne of Glory, as it is stated: “And they saw the God of Israel; and there was under His feet the like of a paved work of sapphire stone, and the like of the very heaven for clearness” (Exodus 24:10). This verse shows that the heavens are similar to sapphire, and it is written: “And above the firmament that was over their heads was the likeness of a throne, as the appearance of a sapphire stone” (Ezekiel 1:26). Therefore, the throne is similar to the heavens. The color of sky blue dye acts as an indication of the bond between the Jewish people and the Divine Presence.
§ The Sages taught: This ḥilazon (snail), which is the source of the sky-blue dye used in ritual fringes, has the following characteristics: Its body resembles the sea, its form resembles that of a fish, it emerges once in seventy years, and with its blood one dyes wool sky-blue for ritual fringes. It is scarce, and therefore it is expensive.