all "lubricating greases" can technically be called a "gel" - a lubricating fluid with a colloidal suspension of a thickener in it that gives the grease its structure. the thickener can be a metal soap (as in the most common greases) but it is also possible to make a grease with more exotic thickeners, eg silica or "claybased" compounds. in the case of silica it is not just ordinary "sand" but a very specific compound with very small dimensions without sharp (and abrasive) edges. the amount used is usually about 5-10 %, the rest of the grease is nearly all base oil or another type of fluid that can be used as a lubricant, often some extra additives (antioxidant, antiwear) are included. the type of thickener has a large influence on the behaviour of the grease - depending on the type used you may end up with a very "thick" grease or a semifluid grease. due to the colloidal structure greases usually have a nonnewtonian character. of course when you add a lot more "thickener" into less fluid the mixture can be very abrasive, as you can see for example in the dredging industry where sand dissolved in water (also a "gel") is pumped through pipes. usually the impellorblades in the centrifugal pumps that are used, only last for about a week and then need to be reconditioned over the weekend...