Talmud for Menachot 87:9

Should a man dye [the zizith of] a ṭallith for himself, they are invalid for religious use if he intended to test [the dye].²⁷The dyeing of zizith must be for the specified purpose of the commandment. The reading ‘to test’ [lenassoth] is preferable to the variant ‘to cover himself’ [lekassoth]. If he dyed them [with an ordinary dye], even four or five times, they are invalid, since the blue [of the zizith] must be dyed only with that of ḥillazon.²⁸A species of mollusc. What does the ḥillazon resemble? In its formation it resembles a fish and [the colour of] its body resembles [the blueness of] the sky. Its appearance is a very rare occurrence²⁹lit. ‘it only rises [from the sea] in seventy years’. and consequently it is very costly.