The Talmud defines the tekhelet dye as derived from the “blood” of a rare amphibious snail known as the chilazon. The mitzvah to wear a thread of that dye on one’s tallit is mentioned multiple times in the Torah. But the tekhelet supply ground to a complete halt following the Muslim conquest of the land of Israel in 638 CE, when Jews were cut off from their local source of the chilazon. Though many theories surfaced over the years, even the most educated Jews had no concrete proof of which modern-day creature the Talmud’s chilazon actually was. Due to that mystery, the mitzvah of tekhelet was largely lost, becoming a source of puzzlement for centuries.Interesting. So someone has figured out how to make bluish-purple dye from powdered snails that could be the ones used in antiquity to make the tekhelet dye. That is encouraging, but not in itself convincing. It seems that some material actually dyed with tekhelet survives from antiquity (see here). What would be convincing if a chemical analysis of that dye showed that the modern reconstructed dye has the same composition. I don't know how close we are to being able to make such comparisons, which ideally would involve non- – or at least minimally – destructive chemical analysis, but this sounds like a desideratum for the future.
Now, thanks in large part to a few enterprising individuals and some enthusiastic rabbis, what appears to be original chilazon has been found—and tekhelet strings are being dyed in a fashion much like the process performed by our ancestors.
Additional background in the last link above, as well as here and here and links.
