Inside the hypobranchial gland of the Murex trunculus the precursors to the tekhelet dye exist as a clear liquid. When these are exposed to air and sunlight in the presence of the enzyme purpurase (which also exists within the gland) these precursors turn into dye. Purpurase quickly decomposes, so the gland must be crushed soon after being taken from the live snail. This is in accordance with the Talmudic passage that the tekhelet is taken from the Chilazon while it is still alive.

The liquid from the trunculus produces a mixture of dibromoindigo (purple) and indigo. These molecules must be put into solution for them to bind tightly to wool. In this state, if dibromoindigo is exposed to ultraviolet light it will transform to indigo, turning the trunculus mixture from purplish-blue to pure blue.