SILICA MINERAL is a term used to describe minerals composed of silicon dioxide, which are found in many different forms. Two of the most common silica minerals are QUARTZ and OPAL, both of which have 6 and 4 letters respectively. QUARTZ is a hard, crystalline mineral that is often used in jewelry and other decorative items, while OPAL is a softer, more colorful mineral that is often used in jewelry and other decorative items.