Solution
For the Secret Ingredient of Number Alchemy, we are pleased to present:
π
The answer to the ultimate question of life, the universe, and everything.
Or, if you will, the ratio of a circle's circumference to its diameter.
Or, if your will, the 16th letter of the Hebrew alphabet.
Or, if you insist, the 3.14th digit of pi.
Or, if you're feeling particularly pedantic, just use the unicode character π.
But let's be real, folks, we all knew it was just going to be 3.14, didn't we?