Gypsum also crystallizes as translucent crystals of selenite. This eliminates water, cubic symmetry crystals and glass. It is often highly decorated with gold, silver, crystals, and feathers. They are made of simple hexagonal ice crystals. An early use of this property of quartz crystals was in phonograph pickups. They are made of simple ice crystals, hexagonal in shape. They exist in form of molecular crystals or amorphous thin films. The urinalysis may show uric acid crystals or amorphous urates. This is one of the most common and simplest shapes found in crystals and minerals. The compound crystallizes as tetragonal birefringent crystals. They come in two forms, liquid crystals and leuco dyes. Fluoride helps prevent decay of a tooth by binding to the hydroxyapatite crystals in enamel. Fluoride helps prevent dental decay by binding to the hydroxyapatite crystals in enamel. Many types of sandpaper use aluminium oxide crystals. It was originally made of microscopic herapathite crystals.