Deep-sea invertebrates, including glass sponges, provide an abundant source of very unusual skeleton structures and inspire investigations aiming to gain a better understanding of biomineralization mechanisms, which can be used to develop new biomimetic based biomaterials. The study focused on the spicules of Hyalonema sieboldi Gray 1835, which are glassy fibres several millimetres in diameter and up to one metre in length - a size which is exceptional for a siliceous biological structure. They exhibit remarkable flexibility, thus giving the impression of being an almost unbreakable glass, which can be easily bent into a full circle. The origin and the possible role of the organic matrix within these silica-containing skeletal formations are still unknown. Here, a new approach is described for the desilification of glass sponge spicules based on a slow etching procedure. To test our hypothesis that an organic matrix within the H. sieboldi spicules is responsible for the extraordinary mechanical properties, structural (SEM, TEM, AFM) and biochemical (HPLC, PAAG-Electrophoresis, MALDI) analyses of these glassy composites and extracted proteinaceous components were performed.