The best replacement for opal is moonstone.

The best alternatives to opal are rated as following:
★★★★★  moonstone
★★★  tourmaline
★★  topaz
★★  opalite
★★  pink tourmaline
★  edrive
★  natural opal
★  ambi
★  synthetic opal
★  black radiance
★  created opal
★  mother of pearl
★  coffeescript
  tourmaline birthstone
  australian opal
  genelec
  jenu
  quartz
  fips
  man made
  real opal

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

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



Google Trends

Scale is based on the average worldwide traffic of opal in all years. Click on graph for more.
opal
moonstone
tourmaline