Defines the type of surface obtained by breaking a crystal in a direction other than that of cleavage. Types include conchoidal, shell-like as in glass and often in gemstones. Also even, uneven and hackly or splintery as in nephrite. Synthetic spinel imitating aquamarine may show cracks at right angles and aquamarine does not. Feldspars cleave and chalcedony does not.
Splintery fracture is seen in nephrite and hematite.
Conchoidal fractures are a strong indicator of glass. I ve seen quartz do it too to some degree. Harder stones will scratch softer ones. The Mohs scale is used for gemstone hardnesses. Mohs Scale 1.