Bio-Gide® is a collagen membrane routinely used in guided bone regeneration. Recent studies have shown that this collagen membrane has osteoconductive properties, meaning that it can support the growth of new bone. However, it has also been observed that the collagen membrane has areas of mineralized fibers which can occur spontaneously and independently of osteoblasts. To better understand how this works, we established a model using minced collagen membranes to reduce the active mineralization of intact collagen membranes in favor of passive mineralization. We thus compared the original intact membrane with a minced collagen membrane in a 5 mm calvarial defect model in Sprague Dawley rats. After three weeks of healing, histology and microcomputed tomography (μCT) were performed. Histological analysis confirmed the osteoconductive properties, with new bone growing inside the intact collagen membrane. However, in minced collagen membranes, the osteoconductive properties were restricted to the defect margins. Interestingly, histology revealed large mineralized areas indicating passive mineralization with no signs of bone formation. In the μCT analysis, the intact collagen membranes caused a higher median mineralized volume (1.5 mm3) compared with the minced group (0.4 mm3), but this lacked significance (p = 0.09). The μCT analysis needs to be interpreted carefully, particularly in defects filled with minced membranes, considering that the mineralized tissue may not necessarily be bone but also the result of passive mineralization. Taken together, the findings suggest that Bio-Gide® collagen membranes support bone formation while also exhibiting potential for passive mineralization.