G'day
I'd make goldworms large creatures like earthworms. Like earthworms, they eat soil. Unlike earthworms, they also eat stone. Their teeth are very hard (carborundum? diamond?) and the power of their agnathous maws enough to bite through rocks. Their integument is very resilient, their bodies elastic and tough, and they are short on vital organs. Their guts contain strong acid, and they are capable of vomiting limited quantities of this up to soften rocks. Goldworms can tunnel through any sort of soil at appreciable speed, but for subsistence they require soil containing organic matter. They have ruined the land they live in by eating most of the soil and casting up mounds of inorganic sand and gravel. Under the surface of blowing sand and sliding gravel is a subsoil absolutely ridded with partially-collapsed worm tunnels. The whole countryside is like a rotten cheese. And the worms are now so hungry that they don't necessarily wait for animals and plants to turn into soil before trying to eat them. But this is not the great danger. The great danger is that great goldworms grow from tiny eggs almost to small to see, and that the goldworm wastes are scattered with dormant eggs. If a party were to cross the barriers of sand and rock into the goldworm waste and come out alive, their clothes would be contaminated with goldworm eggs. And if these got into the soil of another country the worms would spread bringing (after a century or two) total devastation after them.
The opportunity that goldworms bring with them is that their digestions do not affect inorganic matter such as gold, rubies, sapphires, and beryls (though they do digest diamonds, and their digestions destroy active metals such as copper and iron. So when they eat or tunnel through soils containing gemstones or gold they eventually cast it up on the surface (along with softer and more chemically active rocks reduced to sand and powder). The wind blows away the sand and insoluble salts, leaving gold dust, nuggets of gold, and precious stones on the tops of [some of] the mounds of worm-gravel that make up the landscape.
Regards,
Agback