"变成石灰质" can be translated into English as "turn into calcareous matter" or, more contextually appropriately, "calcify" (when referring to the process of becoming calcified or hardening into a limestone-like substance).
"Turn into calcareous matter" directly translates the phrase but may sound a bit technical or verbose in some contexts.
"Calcify" is a more concise and commonly used term in scientific and technical contexts to describe the process of something becoming calcified or hardening due to the deposition of calcium salts (like limestone).
For example:
The shell of the organism gradually turned into calcareous matter over time.
The tissue began to calcify, forming a hard, limestone-like structure.