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 imagemagick
brew install imagemagick@6 && brew link imagemagick@6 --force

imagemagick@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 imagemagick
brew install imagemagick@6
brew link imagemagick@6 --force