At a gemology research center, two clear, colorless minerals were brought in for testing. Both formed six-sided crystals and looked nearly identical, which initially suggested they might share the same properties. Scratch testing revealed a sharp contrast: one fractured easily under a knife blade, while the other resisted scratching even from a steel file. This unexpected difference led the team to consider how internal bonding might explain the variation in hardness despite their similar outward appearance. Which conclusion best explains the difference in hardness between the two minerals?