Otdfbt

This movie was about a town that was being swallowed by the growing black towering rocks that looked like coal. If you touched them you would slowly turn to stone. One child that was affected was placed in an iron lung as a means to cure him.

I would lay odds that you're thinking of The Monolith Monsters (1957):

In the desert outside of San Angelo, California, a huge meteorite crashes and explodes, scattering hundreds of black fragments over a wide area. The next day, Federal geologist Ben Gilbert (Phil Harvey) brings one of the fragments to his office, where he and local newspaper publisher Martin Cochrane (Les Tremayne) examine it. That night, a strong wind blows over a full water container onto the black rock, starting a chemical reaction.

When Dave Miller (Grant Williams), the head of San Angelo's district geological office, returns from a business trip, he finds Ben's corpse in a rock-hard, petrified state and the office's lab damaged by large rock fragments. Dave's girlfriend, teacher Cathy Barrett (Lola Albright), takes her students on a desert field trip; young Ginny Simpson (Linda Scheley) pockets a piece of the black meteorite rock, later washing it in a large tub outside her family's farmhouse. In town Dr. E. J. Reynolds (Richard H. Cutting) performs Ben's autopsy and cannot explain the body's condition; he informs Dave and Police Chief Dan Corey (William Flaherty) the body is being sent to a specialist. Martin returns to the wrecked office with Dave where he recognizes the large fragments as the same type of black rock Ben had been examining.

Cathy joins them, also recognizing the fragments. She goes with the two men to the Simpson farm; they find the farmhouse in ruins under a large pile of black rocks and Ginny's parents dead. The girl is still alive but in a catatonic state. At Dr. Reynolds' request, they rush her to Dr. Steve Hendricks (Harry Jackson) at the California Medical Research Institute in Los Angeles. He later reports that Ginny is slowly turning to stone; her only hope lies with identifying the black rock within eight hours. Dave brings a fragment to his old college professor, Arthur Flanders (Trevor Bardette), who determines that it came from a meteorite. Back at the Simpson farm, both men notice a discoloration in the ground: The black rock is draining something from everything it touches, including people. Later, tests show that silicon is that substance; in humans it is normally just a trace element. Dr. Reynolds explains that research indicates that one possible function of silicon in the human body is to maintain human tissue flexibility. They suddenly realize that the meteorite's absorption of silicon was the cause of Ben's death, Ginny's condition, and the death of her parents; Steve then prepares and administers a silicon solution injection to the girl.

Here is an image of the girl in the iron lung:

Trailer

I think you're talking about The Monolith Monsters. Wikipedia says:

The Monolith Monsters tells the story of a large meteorite that
crashes in a Southern California desert and explodes into hundreds of
black fragments which have strange properties. When those fragments
are exposed to water, they grow very large and tall. The fragments
also begin to slowly petrify some of the inhabitants of a nearby small
town. The story that unfolds becomes one of human survival against an
encroaching unnatural disaster, that if not stopped, could become a
national ecological nightmare that could pose a possible threat to all
of humanity.