Silicon-containing products, such as silicon, silicon carbide and silicon nitride, containing less than 0.01 weight percent total mineral impurities and selectively determined carbon-to-silicon ratios. The products are derived from plant matter, such as rice hulls and rice straw, containing at least three weight percent silica. Methods are provided for making such high purity silicon-containing products by leaching silica-containing plant matter with aqueous sulfuric acid under controlled temperatures, pressures and reaction times to remove minerals and metals while adjusting the mole ratio of fixed carbon to silica, and then thermally treating under controlled conditions to produce the desired product.