The phylum Porifera (sponges; "pore bearing) is divided into three classes, Hexactinellida, Demospongiae, and Calcarea. Calcarea is the oddball of the group, building skeletal elements out of calcium carbonate (like corals and snails) instead of silica. The Hexactinellids (glass sponges) of which I will focus on, are predominately a deep sea group. They are the oldest of the groups originating about 585-720 million years ago during the Snowball Earth period. During this time, soluble calcium carbonate and silica were formed by reaction with atmospheric carbon dioxide. These reactions provided silicic acid in seawater-the necessary starting material for siliceous spicules. In marine environments, silica concentrations are relatively low, thus the need to augment the process with enzymes. Sponges are unique in that silica deposition is mediated by an enzyme (silicatein). Other organism like diatoms do not enzymatically bolster the process. Spicule formation begins within the cell and extruded when it becomes to large (6-8 micrometers). In the deep sea the higher concentrations of of silica/silicon allow for formation of larger sponges (i.e. if your a sponge the deep sea may be the good life). Within the Hexactinellids, one deep-sea species produces spicules over 3m (9.8ft) in length, making them the largest biosilica structures on earth.

From Muller et al. 2007. Fig. 1 Monorhaphis chuni. a Syntype used for the description by Schulze (1904); the specimen was collected in 1899 at a depth of 1,700 m from the deep-sea bottom during the Valdivia expedition in the Somali basin. Size: 45 cm. The animal consists of a giant basal spicule (ba basalia) around which the body of the animal (bo) is arranged. The giant basal spicules are megascleres and anchor M. chuni to the substratum. This specimen is the syntypus determined by Schulze (1904; Museum für Naturkunde, Berlin; ZMB Por 12700). b Higher magnification of the basal part of the giant basal spicule, which attaches the sponge into the ground; the spicule is associated with stony corals (co). c Cross section through the giant basal spicule (8 mm thick), prepared by F.E. Schulze (University of Rostock, Institute of Zoology). d M. chuni in its natural soft bottom habitat of bathyal slopes off New Caledonia (photograph taken by Michel Roux, University of Reims; reproduced with permission). The specimens live at a depth of 800-1,000 m (Roux et al. 1991). In this region, the sponge occurs at a population density of 1-2 individuals per m2. The animals reach sizes of around 1 m in length

Monorhapis chuni was described by Schulze in 1904 based on specimens collected during the first German deep-sea expedition on the RV Valdivia. Little information has been added to Schulze's original description. M. chuni produces an 'anchoring' spicule that can reach 3m with a maximum diameter of 8.5mm (1/3 inch) with silica laid down in concentric rings. The tall stalk allows the sponge to 'reach' into the water column where flows are higher, better for a filter feeder.The final length and girth are finished in the extracellular space.

The creation of that giant spicule is not a simple process. The process occurs through successive deposition of layers (lamella, layers 2-4 show subsequent growth).  The center of the spicule contains a filament (af in figure below) around which the first lamella formed.  Lamella deposition is mediated by silicatein-related proteins (red ellipsoid dots) arranged on the previous lamella and a proteinaceous cage eventually disintegrated (indicated in b2).  The cage is stabilize by lectin (complex molecule of protein and sugur) molecules (yellow dots).  The actual solid silica lamella (pale green) is formed through combination of silica clusters (grey circles).  The concentric arrangement of the silicatein(-related) proteins/lectin associates is proposed to be stabilized by collagen (gray x's)

From Muller et al. 2007. Fig. 11