<p>Bio-inorganic structures have been found to exhibit impressive optical and mechanical properties, such as control of light and/or high fracture strength. Certain species of diatoms (single-celled algae) form siliceous microshells (frustules) with organized structures that affect the transmission of light or fracture strengths. It has been found that <i>Coscinodiscus wailesii</i> diatoms have frustules with a quasi-regular hexagonal pattern of pores, which act as micro-lenses. In terms of mechanical strength, <i>Fragilariopsis kerguelensis</i> diatom SiO₂ frustules have been observed to exhibit impressive compressive and tensile fracture stress values. In this study, shape-preserving chemical conversion (using gas/solid reactions) is used to transform biogenic structures (diatom frustules) into high IR refractive index or ultrahigh specific strength materials. High-fidelity MgO/Si, Mg₂Si, Ca₂Si, MgO/Ti, and Ti replicas are successfully synthesized and characterized by SEM, EDX, XRD, and TEM. Focal point imaging experiments are used to show that focusing behavior of MgO/Si and Mg₂Si replicas can be enhanced in the IR range upon conversion into higher index replicas. Mechanical properties of SiO₂ frustules, MgO/Ti replicas, and Ti replicas have been measured by using in-situ and ex-situ indentation, which revealed that the mechanical properties can be enhanced by the shape-preserved chemical conversion of Bio-inorganic structures.</p><p><br></p>