Written and first recorded by Dick Feller (1975).
Hit version by John Denver (US #36/C&W #6 1981).
From the wiki: “‘Some Days Are Diamonds (Some Days Are Stone)’ was written by singer-songwriter Dick Feller, and was quite different from the humorous and novelty songs he was best known for writing (e.g., ‘The Night Miss Nancy Ann’s Hotel for Single Girls Burned Down’, ‘Lord, Mr. Ford’, ‘Makin’ the Best of a Bad Situation’). Feller was first to record and release the song, in 1976, but his version failed to chart.
“‘Some Days are Diamonds …’ was later covered by John Denver, on his 1981 album Some Days Are Diamonds. Released in May 1981 as the album’s first promotional single, Denver’s version peaked at #6 on the Billboard Hot Country Singles chart and #36 on the Billboard Hot 100.”