Learn Portuguese (Portugal): Ele cravou o diamante no metal, de forma que a joia é valiosa. - He set the diamond in the metal, so the jewel is valuable.

This is the core of the current lesson! When de forma que introduces a factual consequence or an actual result, it triggers the indicative mood. If it were expressing an intended purpose, meaning he set it in a specific way in order for it to be valuable, it would require the subjunctive seja. Here, the jewel being valuable is just a stated fact about the outcome.

Are de forma que and de modo que completely interchangeable here?

Yes, they mean exactly the same thing and function identically. You could easily say de modo que a joia é valiosa to express the exact same consequence.

What does the verb cravar mean in broader contexts outside of making jewelry?

While here it refers to setting a precious stone, cravar generally means to drive into, to nail, or to stick in. For example, you can cravar a stake into the ground, or use the expression cravar os olhos to describe staring fixedly or piercingly at someone.

Why doesn't the word joia have an accent mark?

If you read older texts, you will see it spelled jóia. The 1990 Orthographic Agreement removed the acute accent from the open oi diphthong in words stressed on the second-to-last syllable. Other common words that lost their accent for the exact same reason include ideia and boia.