The best replacement for obsidian is onyx.

The best alternatives to obsidian are rated as following:
★★★★★  onyx
★★  flint
★★  coal
★  luminary
★  bethesda
★  sundown
★  pumice
★  glacial malphite
★  diamond harder
  malbec black
  bioware
  slag
  gabbro
  tnt
  stargazer black
  stargazer
  hematite
  obsidian

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

Other search terms related to obsidian and their ratings are:
★★★★★ obsidian
You have in mind a companion for obsidian and it is not yet here? Propose!



Google Trends

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