Rashi: Hashem put a limit for it.

Malbim: Dear metals are hidden in the ground. A certain rock reveals the sources, and dispels this darkness (refer to 28:3:2:4).

Rashi #1: I heard, the rock from which darkness comes - it is the source of punishments. It is called Even, like one who bangs his foot on a rock, he has pain.

Rashi #2: This is like "Kav Sohu v'Avnei Vohu", "Di mi'Tura Isgezeres Even..."

Ibn Ezra: This is the land, which is the primary darkness. It is also what darkens the moon during a lunar eclipse, in the middle of the month when it is even with the head of Teli or the tail.

Malbim: Even though the sources of dear metals are hidden in the ground, there is a dark rock that reveals the sources, like a magnet ¹ .