Coating subwavelength-scale pinnacles/thorns on surfaces usually results in antireflection, known as "moth-eye effect". However, fabrication of such coatings is often complicated and expensive. Herein, we present a bottom-up approach for forming a moth-eye-like structure on Si by directly growing layered Mg silicate using a one-step process. When an aqueous solution containing LiF, MgCl2, and urea is heated at 150 °C in the presence of Si, fine crystals of the layered silicate completely cover the Si surface. The resulting thorn-like structures reduce the reflectance of Si in the visible-wavelength range, exhibiting a graded-refractive index profile from air to the Si substrate. The antireflection feature is observed when the height of the thorns is 0.1 μm, which is equivalent to the crystal size of Mg silicate and is influenced by the heating temperature. The heating period is optimized to be 48 h to avoid coprecipitation of light-scattering fine particles, such as amorphous silica and Mg silicate, in excess quantities.