The various physical and visual attributes of a particular rock sample that can be ascertained by touch (or the illusion of touch), such as coarseness, smoothness, grittiness, sharpness, its crystalline quality, etc.
I did this in my geology A level. Part of it involved putting small samples in between your teeth to get a definite texture between sand/mud/claystones. Some could be really gritty where as some we so fine you barely feel anything.