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If any properties are unavailable the game will use placeholders. However, if those properties happen to be strength, it would be best to estimate to the best of your ability, such as by using a similar material (E.G iron for cobalt). Even if you can find only one elastic modulus, this is no problem; Poisson's ratio can only be between -1 and 0.5 in DF terms, and a general rule of thumb can be followed: metals 0.3, woods 0.45, rock 0.25. Young's modulus tends to be available, but if it isn't... guess.
Materials in dwarf fortress are made with the assumption that IMPACT values should be equal to COMPRESSIVE, which is incorrect; YIELD and FRACTURE should be equal to BENDING or COMPRESSIVE, whichever's less (which is always BENDING in vanilla DF). However, forcing this on whoever makes the materials would be dumb, since it is the game that we're talking about. So:
Realistic IMPACT values: