Mohs Hardness of Common Objects
fingernail 2 to 2.5
copper 3
nail 4
glass 5.5
knife blade 5 to 6.5
steel file 6.5
streak plate 6.5 to 7
quartz 7
https://geology.com/minerals/mohs-hardness-scale.shtml
Also I find it quite funny how you know the brand and the exact hardness that blade carries. From what you know, he maybe even used a different knife for the iphone 6s video and changed it up. He is a youtuber for Gods sake. ;)
Also a material with hardness of 7 will scratch deeper and easier (with less pressure) a softer glass than a hard one. So while quartz will scratch every screen:
1) it will scratch DEEPER the softer glass than the hard one
2) hard glass can get away without dmg if the pressure is not that strong and soft material is on the other side of it
3) or hard glass will get away with way less dmg than the softer glass that will scratch deep as **** even from the smallest contact without pressure
Facts. The same as how the 7 pick scratches the screen a lot deeper and easier than 6. The bigger difference between the two materials - the bigger the damage is. So that 7 will scratch a lot worse and easier glass that is 5.2 vs glass that is 5.9.