The best replacement for onyx is cocktail.

The best alternatives to onyx are rated as following:
★★★★★  cocktail
★★★  cleanmymac
★★★  jet
★★  obsidian
★★  maintenance
★★  mackeeper
★★  ccleaner
★★  raquel
★★  caldera
★★  onyx
★  cultured marble
★  amethyst
★  nicole
★  agate
★  geodude
★  applejack
★  dragon bolts
★  silver pigeon
★  stratus 7
★  swanstone
★  hematite
★  ice clean
★  black diamond
  mainmenu
  rane ms1b
  jade
  black spinel
  xdr2
  iceclean
  alabaster
  presonus firepod
  dynafit
  firepod
  magic city
  vlz

Your alternative for onyx is not listed? Suggest!
Speaking about onyx, one probably shouldn't forget about biohazard.

Other search terms related to onyx and their ratings are:
★★★★★ biohazard
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Google Trends

Scale is based on the average worldwide traffic of onyx in all years. Click on graph for more.
onyx
cocktail
cleanmymac