Isatis tinctoria
The thread of Tekhelet is a blue-violet or blue thread, which, according to the traditions of Rabbinic Judaism, is to be dyed with a specific kind of dye derived from a mollusc (notably the "Hexaplex trunculus" sea snail). Due to a number of factors, including Rome outlawing the use of Tekhelet by commoners, the source and practice of using a Tekhelet thread in Tzitzit was lost for most Rabbinic Jews. Their Tzitziyot are usually all white. Karaite Jews believe that the importance of Tekhelet is that the color of thread is blue-violet and it may be produced from any source, including synthetic industrial dyes, except impure (a state mostly overlapping unkosher) marine creatures, rather than insist on a specific dye. Therefore, they believe that the rabbinic tradition of relying on a dye from a mollusc is incorrect. They suggest that the source of the dye was indigo or "Isatis tinctoria".