Chondroblastic osteosarcoma, light micrograph. Osteosarcoma is a malignant neoplasm in which the tumour cells produce immature bone or osteoid. Based on the morphology, the most common types of osteosarcomas are osteoblastic, chondroblastic, and fibroblastic. These patterns often coexist although their relative proportions vary from case to case. This is a chondroblastic osteosarcoma. The neoplastic cells are producing osteoid (homogenous pink areas) as well as cartilage (bluish-grey zones). Haematoxylin and eosin stain.