A snippet of my edited material_template_default.txt
[MATERIAL_TEMPLATE:HORN_TEMPLATE]
[STATE_COLOR:ALL_SOLID:GRAY]
[STATE_NAME:ALL_SOLID:horn]
[STATE_ADJ:ALL_SOLID:horn]
[STATE_COLOR:LIQUID:GRAY]
[STATE_NAME:LIQUID:n/a]
[STATE_ADJ:LIQUID:n/a]
[STATE_COLOR:GAS:GRAY]
[STATE_NAME:GAS:n/a]
[STATE_ADJ:GAS:n/a]
[DISPLAY_COLOR:7:0:0]
[MATERIAL_VALUE:1]
[SPEC_HEAT:420]
[IGNITE_POINT:10508]
[MELTING_POINT:NONE]
[BOILING_POINT:NONE]
[HEATDAM_POINT:10250]
[COLDDAM_POINT:9900]
[MAT_FIXED_TEMP:NONE]
[SOLID_DENSITY:500]
[LIQUID_DENSITY:NONE]
[MOLAR_MASS:NONE]
[IMPACT_YIELD:200000] used bone for all of these, no data
[IMPACT_FRACTURE:200000]
[IMPACT_STRAIN_AT_YIELD:100]
[COMPRESSIVE_YIELD:200000]
[COMPRESSIVE_FRACTURE:200000]
[COMPRESSIVE_STRAIN_AT_YIELD:100]
[TENSILE_YIELD:115000]
[TENSILE_FRACTURE:130000]
[TENSILE_STRAIN_AT_YIELD:100]
[TORSION_YIELD:115000]
[TORSION_FRACTURE:130000]
[TORSION_STRAIN_AT_YIELD:100]
[SHEAR_YIELD:115000]
[SHEAR_FRACTURE:130000]
[SHEAR_STRAIN_AT_YIELD:100]
[BENDING_YIELD:115000]
[BENDING_FRACTURE:130000]
[BENDING_STRAIN_AT_YIELD:100]
[MAX_EDGE:1000]
[ABSORPTION:100]
[IMPLIES_ANIMAL_KILL]
[SHELL]
[HORN]
[ITEMS_HARD]
As you can see at the bottom i have just added [SHELL] into the mix.
Worked for me, keep in mind you need to change the material template file in
both your main folder and saved game folder.
Also it only applies to NEW items, so as I said make sure you choose an item which you can 'create' after making the changes.
I have not tried this with stones or metal bars but in theory it should work. I just found it convenient to set on an item which shares the same stockpile, is near useless, and can be created easily by slaughtering stuff.