RMagick installation: Can't find MagickWand.h
To solve the issue in Mac OSX Sierra, High Sierra, El Capitan, Mojave, Catalina, and Big Sur, you can do the following:
brew unlink imagemagickbrew install imagemagick@6 && brew link imagemagick@6 --forceimagemagick@6 is keg-only, so you'll need to force linking.
I had this same issue with ImageMagick 7.0.4-4. The proper fix is to install imagemagick@6. I found https://github.com/Homebrew/homebrew-core/pull/8756 useful if you accidentally deleted your imagemagick@6 or never had it installed in the first place.
macOs Sierra:
brew uninstall imagemagickbrew install imagemagick@6brew link imagemagick@6 --force