Among several species of the Gramineae, Avena sativa L. (oats) is an important opal-accumulator plant in Victoria. In it, amorphous silica has been precipitated as very abundant microscopic opal phytoliths of various shapes, among which hook-shaped forms are common in alternating rows of epidermal cells. Examples studied in detail were observed within and isolatd from oats grown on basaltic soil on "Barewood Farm", approximately 2 miles north of Clarkefield in the Romsey district of South Central Victoria. They are variable in shape and size, and in their relationships to adjacent rod-like opal phytoliths that infill the lumen of several of the subepidermal cells. Each hook-shaped opal phytolith consists typically of a solid core of opal enveloped distally by a more delicate cone-like sheath of opal. The opal of the sheath is hyaline and transparent compared with the thicker, generally translucent core. Refractive index variations reveal differences between core and sheath, and between the several growth layers of slightly different composition in the sheath. These differences reflect variable ratios of silica to water in the opal of the core, the sheath, and its several layers. The hook-shaped opal phytoliths, like all other shape types secreted in short-lived plants, must have developed rapidly, because processes of opal precipitation and solidification were completed within the compass of one growth season.