:-)
Quartz is indeed quite hard.
Gypsum - as we used to say when drilling oil wells through it - is "soft as shite".
(calcite not much better, calcretes as Nick pointed out is a calcite-cemented soil so may be overall harder)
There are always plenty of possibilities........ (I'm putting my money - for the outcrops at least - on feldspars - just coz of the way they weathered)
C'mon Opportunity! Get drillin'!