What common mineral's name (technically silica or SiO2) derives from old Polish-German 'kwardy', meaning 'hard'?
Silica gel Silica gel Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other liquids, or may be filled by gas or vacuum. In the latter case, the material is properly called silica xerogel. Silica xerogel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a desiccant. It is hard and translucent, but considerably softer than massive silica glass or quartz; and remains hard
Hard-clad silica optical fiber Hard-clad silica optical fiber Hard-clad silica (HCS) or polymer-clad fiber (PCF) is an optical fiber with a core of silica glass (diameter: 200 µm) and an optical cladding made of special plastic (diameter: 230 µm). In contrast to all-silica fiber, the core and cladding can be separated from each other. Due to their medium bandwidths and transmission rates of less than 100 MBit/s, HCS fibers are suitable for distances of up to 2 km, e.g. in local networks in buildings and industry. Generally, the following applies: The higher the attenuation, the shorter the distance. For comparison, plastic optical fibers (POF)