Techelet is specifically wool dyed with the chilazon's blood. The source matters — visually identical blue from other sources (indigo, plants) is invalid.
Question 2
Can you use indigo dye instead of chilazon dye if the color matches?
The Rambam is clear: the source matters, not just the color. Indigo may look identical but is invalid for techelet.
Question 3
What color should techelet resemble?
The Rambam describes techelet as resembling "the sky at noon" — a clear, bright blue. Darker or greener shades don't qualify.
Question 4
How many of the eight tzitzit threads should be techelet?
One of the eight hanging strands is techelet; the rest are white. The single blue thread is meant to catch the eye and trigger remembrance of the commandments.
Question 5
Why was techelet lost for centuries?
The chilazon's identity was lost, making authentic techelet production impossible. The obligation continued with all-white tzitzit. Modern scholarship has proposed the Murex trunculus snail as the chilazon.