Truth and falsehood.

[13:17] He sends down water (rain) from the sky, and the valleys flow according to their measure, but the flood bears away the foam that mounts up to the surface, and (also) from that (ore) which they heat in the fire in order to make ornaments or utensils, rises a foam like unto it, thus does Allah (by parables) show forth truth and falsehood. Then, as for the foam it passes away as scum upon the banks, while that which is for the good of mankind remains in the earth. Thus Allah sets forth parables (for the truth and falsehood, i.e. Belief and disbelief).

ENGLISH TAFSIR  - TAFSIR IBN ABBAS

Allah then gave a simile of the Truth and falsehood, saying: (He sendeth down water from the sky) He says: He sent Gabriel with the Qur'an and explained therein the Truth and falsehood, (so that valleys flow according to their measure) so the illumined hearts bore the Truth in proportion of their size and light, (and the flood beareth) the darkened hearts bore ((on its surface) swelling foam) much falsehood due to their whims (from that which they felt in the Fire) this is another simile. He says: and of that which you cast in the fire of gold and silver contains adulteration just as the foam of the sea does have salt (in order to make ornaments) which you were. Allah says here: the Truth is like gold and silver which benefit people, and falsehood is like the impurities which are derived from the casting of gold and silver, which are of no benefit. Likewise falsehood benefits no one (and tools) ore and copper (riseth a foam like unto it) He says: these also have impurities, i.e. just as the foam of the sea. This is another simile. He says: the Truth is like ore and copper from which people benefit, just as people benefit from the Truth. And falsehood is like the impurities resulting from the casting of ore and copper, which are of no benefit, just as falsehood is of no benefit to anyone, (thus Allah coineth (the similitude of)) thus Allah explains (the true and the false. Then, as for the foam, it passeth away as scum upon the banks) it goes just as it comes: it is of no benefit to its owner, (while, as for that which is of use to mankind) pure water, gold, silver, ore and copper, (it remaineth in the earth) it is useful just as the Truth is. (Thus Allah coineth the similitudes) Allah explains the similes of the Truth and falsehood.