“I can hardly write a piece without stones in it,” the composer Steve Heitzeg said, laughing. Seated in a rehearsal room in the basement of Orchestra Hall, Heitzeg was explaining his use of stones in the opening measures of his new cello concerto, which the Minnesota Orchestra will premiere in concerts Friday and Saturday.

In the score he asks that the stones be “randomly” rubbed and tapped together by a percussionist, the effect of which, if it can be heard at all, would be subtle. And, presumably, just about any collection of stones would suffice.

Heitzeg, it should be noted, doesn’t intend his stone-rubbing to be a joke. He incorporates “natural” instruments — fallen tree branches, whale bones, sea glass shards, along with found objects such as plowshares — into many of the 150 or so

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