Yes, magnetars are considerably more rare than black holes and considerably more interesting to study in terms of raw horsepower. Imagine a type-2 civilization using them as engines or launchers for spacecraft to zip around the galaxy.
The can produce Earth masses of gold, but of course not in the form of "chunks". The gold starts as atoms in a highly ionized gas and only much later cools enough to form solids (dust).
https://en.wikipedia.org/wiki/Magnetar
"A magnetar's 10^10 tesla field, by contrast, has an energy density of 4.0×1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead."