Something that provides a way to gain knowledge

Solution 1:

Heuristic serves both as noun and adjective for this.

As adjective:

helping to learn; guiding in discovery or investigation (Collins).

Of, relating to, or enabling discovery or problem-solving, esp. through relatively unstructured methods such as experimentation, evaluation, trial and error, etc. (OED)

As a (count) noun, it refers to something that is heuristic in the adjectival sense, especially any more or less systematic process or method for generating ideas (which function belongs to Invention, the first of the five canons of classical rhetoric, heuresis in Greek or inventio in Latin).

Solution 2:

You might consider beacon as a suitable metaphor for this.

Symmetries are the beacons of knowledge.

This would be a concise and effective way to express your idea.

Solution 3:

I believe you are referring to a sign-post:

noun 1. a post bearing a sign that gives information or guidance. 2. any immediately perceptible indication, obvious clue, etc.

Originally and literally, this is a metal or wooden rod driven vertically into the ground, in order to hold a sign. Said sign gives information about which direction to turn for the nearest gas station, or to get to a particular town. From there, is has developed a figurative meaning as anything that gives guidance or clues. For example, "When my doctor gave me the lab results, it was a sign-post that I needed to change my dietary habits."

In response to your edit: You could say something like "Symmetry is a sign-post to the wonders of our world."

Solution 4:

I suggest edifying,

• (adj.) providing moral or intellectual instruction

  [Google Definition]

• Instruct or improve (someone) morally or intellectually

  [ODO]

Usage:

You could perhaps say,

Symmetry is an edifying conception.

Note that it is synonymous with illuminating, uplifting etc.