Paul Goldstein wrote:

Do you know the source of the dye that was used to dye the techeilat of the tztisit the color blue. I thought it was from the snail "Chilazon", but my daughter told me that her Hebrew School teacher said it was from a fish.

Dear Paul,

You're both right! The commentators call the Chilazon a 'fish' since it lives in the water. It appears, though, that the Chilazon is not a normal 'fish,' but has a shell and is actually a type of snail. It ascends to the land very rarely, only once every 70 years.

Over time, the exact identity of Chilazon became obscure for various reasons: One reason was its extreme rarity. Because of this, its cost was prohibitive - 10 to 20 times its weight in gold. Another reason: In the 4th century Rome decreed that only 'blue-blooded' royalty could wear Techelet. Although some modern-day Torah scholars maintain that they have re-discovered the Chilazon, their opinion is not universally accepted.

Sources:

• Rabbi Y.M. Tuchashinski, "The Holy City & Temple" 5:5-6.
• Rabbi Y.H. Hertzog, 'Royal Purple & Biblical Blue' ch.11.