In order to reduce the environmental impact of the construction sector, research has been led to use seashell co-products in cementitious materials, showing they could bring better durability properties or be used as limestone filler. First results also showed higher porosity and lower thermal conductivity with concrete made with oyster shell aggregates. To better understand physical and chemical modifications brought by seashell co-products, it is essential to have a good knowledge of seashell microstructure and composition. This study showed that oyster shell microstructure is made of two different phases mostly made of calcite: a foliated and a chalky porous structure. The chalky porous structure contains more chemical elements than foliated structure and a porosity about 35% based on SEM and µ-tomography results.