NPC diamond piracy is profitable now that LTD prices have skyrocketed in the blackmarket.
As already suggested check WiggyB videos that explain the basic ABC of the whole process.
First go to EDDB and search for Agricultural Systems in Boom State and with Population above 1bn. Some people prefer to add Anarchy government as well so they can pirate freely without having to wait for the target to high wake away from secured systems.
Scan the target, if it has a mining laser then it has diamonds. Targets vary from Adders all the way up to T9s. Again, if it has a mining laser then it has diamonds. (and for me it is a target)
Once you interdict the target there are different techniques to fetch the cargo from the benefactor:
1) shield tanking
2) disabling drives and "bumping" the target to a stop
3) disabling the FSD, eventual point defence and all hardpoints, this will make the target "surrender" (my favourite technique)
The usual technique from 2.4 (disabling the powerplant) does not longer work, FDev is aware of the thing but so far no patch has been issued, so for now stick to one of the above methods.
Personally I do a mix of 1) and 3), it depends on the target.
For further reference:
https://www.youtube.com/watch?v=nHKBhttDJ_0
As for the "high value couriers" which go by the name of Private Courier Service, they are rare but so far I saw them spawning exclusively on Agricultural, High Pop, Boom and secured systems (medium to high security). They carry very valuable stuff but not much (30t at most).
Honestly I pirate them only if they really happen to come by me because they spawn rarely.
And on the profit discussion.
My pirate python carries 128t load of diamonds that sold on the black market generates approx 11mln. (this can be filled in 30minutes, perhaps less)
My 320t pirate cutter generates me approximately 26mln on a full load. (approx 1hr of playing time)