Diamond-bearing kimberlite sample collected by the author from the Sloan 2 kimberlite, Colorado.
Note the large, emerald green, chromian diopside megacryst in the kimberlite matrix.

Many kimberlites occur in the US as well as Canada. And essentially all of the kimberlites contain chromian diopside - some are much richer in this emerald green gem than others, but for the most part, diamond operations retain diamonds, but toss the chromian diopside and gem-quality garnets into their waste piles. Although, no research has tried to determine the amount of chromian diopside that occurs in kimberlite, many diamond pipes likely have millions (if not hundreds of millions) of carats of the diopside and garnet gems. 

Some kimberlites, lamproites, lamprophyres and detrital diamonds within the Wyoming craton
(from Hausel, 1998).

Map of US showing locations of kimberlite, lamproite, lamprophyre and other anomalous minerals and rocks of interest. Not all are shown a the map, such as Kilborne Hole in southern New Mexico.