Learn Dutch: De geslepen edelsteen is hem te klein. - The cut gemstone is too small for him.

This is an example of dative complementation. With evaluative adjectives (especially those modified by te, meaning 'too'), Dutch often drops the preposition voor (for) and places the object pronoun (hem) directly before the adjective phrase. While is te klein voor hem is also grammatically correct, is hem te klein is smoother and highly typical for Dutch.

Edelsteen is a de word, so shouldn't the adjective get an -e ending? Why isn't it geslepene?

Geslepen (cut/polished) is the past participle of the strong verb slijpen. Because it already ends in -en, it never gets an extra -e ending when used as an adjective before a noun. This is the same rule that applies to other strong past participles (like gebroken) and adjectives describing materials (like houten or gouden).

If I wanted to use a name instead of the pronoun hem, would the word order change?

The structure remains exactly the same. You would say De geslepen edelsteen is Peter te klein. Just like the pronoun, the dative noun (the person experiencing the evaluation) sits right before the adjective phrase it complements.

Can I use this dative structure with any adjective?