The internal structure of a mineral is dependent on the arrangement on the atomic level. The external shape can be influenced by several factors including the internal arrangement of atoms, mode of formation and environment of crystallization. Twinning is common to some minerals. Twinning occurs when there is a small defect in the atomic bonding of the mineral causing the mineral to deviate from it's perfect form. This deviation results in penetration, repeated and contact mineral twins.
Sulfides are typically opaque and characteristically colored with distinctive streaks.
Those that are nonopaque have extremely high refractive indices and transmit light only on their edges. The majority of mineral ores are sulfides. Sulfides are typically opaque, colored with distinctive streaks. Sulfosalts are usually found as minor minerals in hydrothermal deposits and commonly contain lead, silver or copper. They are not commonly used as ores.