It was a rare gift that was given to him. The color was very rare for an orchid.
She rarely frowns. The book is rarely opened.
My brother rarely goes to the cinema. She is rarely sick
rarely
I rarely eat fast food because I prefer healthier options.
Sometimes. BUT it depends on the sentence you had in mind. Very rarely the sentence works with but as the starting word.
The word rarely is the adverb, as it describes when or where the poison ivy grows.
Adverb
Conviction of a federal offense rarely (if ever) results in a deferred sentence.
Note that the word is rarely used in English.
Demonology is a rarely seen field of scientific study.
Rarely have I seen an animal with fur.
We rarely buy vegetables in the late summer because we grow our own.