硅藻土是一种生物化学沉积岩；由硅藻的细胞壁沉积而成；淡黄色或浅灰色，质地软而轻，可轻易的磨成粉末；密度低、多孔隙、有粗糙感，有极强的吸水性。烘箱干燥的硅藻土的典型的化学组成为80〜90％的二氧化硅，用2〜4％的氧化铝（归因大多以粘土矿物）和0.5至2％的铁氧化物。

硅藻土是热、声和电的不良导体，因此可作轻质、绝缘、隔音材料；又用作游泳池的过滤物质、打磨物质、液体的吸附物、阿尔弗雷德·诺贝尔所发明硝化甘油炸药中硝化甘油的吸附物、杀虫物质等等。硅藻土也可当作绝缘物质。

英语的名字称之diatomite是硅藻化石的堆积物主要成分为SiO₂，硅藻在海里或是湖里大量生长后死亡，硅藻死体的有机物在漫长的时间中被渐渐分离最后以SiO₂的形式留存下来，这就是现在人们称之为硅藻土，大多数是来源于白垩纪以后的地层。

Diatomaceous earth /ˌdaɪ.ətəˌmeɪʃəs ˈɜːrθ/, also known as D.E., diatomite, or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 3 micrometres to more than 1 millimetre, but typically 10 to 200 micrometres. Depending on the granularity, this powder can have an abrasive feel, similar to pumice powder, and has a low density as a result of its high porosity. The typical chemical composition of oven-dried diatomaceous earth is 80 to 90% silica, with 2 to 4% alumina (attributed mostly to clay minerals) and 0.5 to 2% iron oxide.