Learn French: Le granit, c'est très compact.

Both are correct, but using c'est turns it into a broad, general statement. In everyday French, it is very common to state the topic first (Le granit), pause, and then describe it as a general category using c'est.

If we swapped Le granit for a feminine noun like La pierre (stone), would compact become feminine?

No, it stays exactly the same. When you use c'est to make a generalization, the adjective that follows is always masculine singular, regardless of the gender or number of the noun being discussed. You would say La pierre, c'est très compact.

Why is there a comma after granit?

The comma represents a natural pause in spoken French. This sentence uses a structure called dislocation, where you announce the topic first (Le granit), then use a pronoun (c') to actually build the sentence. The comma separates the announced topic from the main clause.

Do I pronounce the final 't' in granit?

You can, and many native speakers do! While a final 't' is usually silent in French, granit is an exception where the 't' is frequently pronounced, though keeping it silent is also correct.