It was and still is, the most expensive dye on earth.
 
Murex trunculus sea snails can only be collected by diving and hand-searching on rocky coastlines. A special gland inside the snail contains the valuable dyestuff which is removed and dried for further use.  This dyestuff is not just collected for the color, the glands are a by-product of the snails that are eaten as a fried snack in the Mediterranean.
 
Tyrian purple and tekhelet are two colors that can be produced with the murex trunculus, and for this you have to make a reduced vat, just like with indigo. If you are not doing anything special to your vat, it will give purple on wool and blue on silk.
 
We currently have 10 grams of murex glands in a metal container, which comes with instructions on how to prepare your vat of two liters. 
 
I have written an extensive blog on the subject of tekhelet blue and tyrian purple that you can read here.