Since the Virtual Rock only makes quiet clicking noises when the buttons are pressed we thought it best not to have a volume control. If you listen carefully when the rock is very happy you can here it singing quietly to itself. We aren't sure if this answer is correct for all Virtual Rocks, so let us know if your Virtual Rock is different. Rocks generally are heavy, with the exception of Pebble and Sedimentary who get ill if they are too heavy. Most baby rocks start out at 45 grams.
Some rocks evolve when there weight reaches a certain threshold like Granite, for example.
Remember it is not good to have a rock that is either too heavy or too light. Virtual Rock sometimes gets sick. Sometimes it's because you forgot to take care of it. Other times it just gets sick for no reason, just like people get the flu. Do not worry if Virtual Rock gets sick for no reason.