A suggestion for detecting gold inside black sand...
Most black sand does NOT contain microscopic gold in economic quantities, or else the gold price would take a tumble.
But sure, sometimes it DOES and then the baby is thrown out with the tailings and the bathwater.
We use the following trick in Mongolia.
1: notice that gold is remarkably soft and malleable.
2: notice that black sands are often brittle and not malleable (e.g. magnetite).
3: put dry black sand on a sheet of clean steel.
4: beat the black sand with a small hammer.
5: the black sand grains shatter.
6: any gold is released.
7: any gold is either tiny and visible only with a x40 to x100 stereo-zoom microscope, or is big enough to be beaten flat and rendered large enough to be seen by eye in bright light.
8: use a feather to gently scrape away the shattered black sand.
9: gently blow to gently blow away more shattered black sand.
Cheap, quick and interesting.
Usually there is no gold, but then when there is....!!