Jugs are incut or have a very large edge so they are good for overhanging walls. These are great for mantles, lunging off of, or lunging to because they are so big, you just can't fall off them. River Sandstone Like the name says, we get these out of a river. River rounded rocks are basically round or egg-shaped with little or no features of the original rock left. These shapes aren't particularly realistic but are great for strength training when you continually go up and down the same holds.
Most river rock is polished and quite slick.
The sandstone we use is an exception-it just doesn't polish and it has a fairly good grip. Very smooth river polished rock might be useful if you spend 4 hours a day training and climb 5.13. However, we've found that most chmbers want holds that have a fair amount of friction. The sandstone we use is somewhat porous so it tends to soak up sweat which gives it an advantage over plastic holds. These are rounded versions of our Sandstone Micros.