Final carats tally comp., page-344

Now if you understand the difference now think of how they can be collect The primary rock needs chisels and jackhammers and some blasting thus damaging a lot of rubies
The secondary rubies can be scrapped up with the soft medium around them. The locals do this with shovels
and are supported buy MUS to make it easier for them by scraping the top soil of and by providing them the screening equipment.
Those same machines scrape up the secondary deposit and they go into MUS larger plant for screening and separating.
The labor costs to collect the secondary rubies is a lot cheaper than all the work needed on the primary