As nouns the difference between nopal and opal
is that
nopal is a prickly pear cactus from the genus
Opuntia, especially species: Opuntia cochinellifera while
opal is a mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO
2·nH
2O.
As a proper noun Opal is
{{given name|female|from=English}} from the precious stone, invented in the nineteenth century.