The incident happened at the firm’s Bromsgrove quarry in Worcestershire.
The Health and Safety Executive said an employee had been working a power-operated machine when the lump of sandstone fell about 6m and struck him on the head.
No head protection was being worn by the employee and he received serious injuries.
The incident is one of more than 2,700 quarry incidents reported to the HSE since 2000 – 21 of which have been fatal.
Investigating inspector Ian Williamson said: "Assessing risks and implementing safe systems of work, particularly on moving machinery, often only requires simple control measures but, in this case, screens, which may have prevented the boulder from entering the machine, were not sufficiently maintained.
"It also obvious that workers should wear suitable head protection in an environment where there are large, heavy, materials swiftly moving along at high level.
The firm was ordered to pay £3,520 in costs.