VLF IB machines, even multifrequency ones have difficulties in black sand.
PI machines on the other hand, tend to see through it easily.
If you can stand the French language soundtrack, this long video shows a prototype of the new Manta detector - a PI acquired and being developed for production by Fisher.
Starting about 4 minutes in, the Manta detects a gold band below a bag of black sand - a Minelab Sovereign sees nothing.
https://m.youtube.com/watch?v=G8sdp4RG73g&feature=youtu.be
A PI with iron ID and depth greater than any VLF will shake up the beach scene - soon, I hope.