3DM and STL are not two versions of the same file
The 3DM is the source file of the CAD work: it lets you keep working, edit and make changes. The STL is meant mainly for passing geometry to production, describing the surfaces as a mesh - a network of triangles. Put simply: the 3DM is the file you go back to; the STL is the file you send to production.
What actually happens when an STL is created?
When an STL is created, the surfaces of the CAD model are translated into a network of triangles. The better that network describes the original geometry, the more faithfully it can represent curves, transitions and details. It is important to separate the level of detail of the mesh from the resolution of the printer - they are two different variables.
Can’t you just send the 3DM to the print house?
Workflows vary, but in a well-organized process it is right to separate the source file from the production file. When a pre-checked STL is delivered, it is clearer which geometry was approved and sent to print - and the source file stays available for editing.
Why keep the 3DM file as well?
This production run is not necessarily the end of the model’s life. In the future you may need to change a size, fit a different stone or create a variation. Returning to a clean, editable CAD model is very different from trying to edit a mesh that was made for production.
Is every STL ready to print?
No. The existence of an STL does not mean the model behind it is right for production. Production-ready is not a file format but a condition in which the model has passed the necessary checks. Converting to STL does not turn an incorrect model into one that is ready for production.