What is Tekhelet?

Tekhelet is one of the colors mentioned in the Torah, traditionally associated with a shade of blue. It is mentioned frequently alongside gold, silver and silk as a precious commodity. There is a Biblical commandment to tie a thread of Tekhelet around the tzitzit (fringes) of cornered garments. In addition, Tekhelet is required in the garments of the High Priest, as well as for the coverings of the holy vessels.