Stones in the wild: A Pet Rock study

Stones in the wild: A Pet Rock study

Mar 01, 2010

Remember Pet Rocks? They might have been a short-lived fad, but I think they're still alive and well in the U.S. Some clients who live with pet rocks reside in rural areas, but most dwell in urban locations. Because these people have never had pets, they're ignorant regarding most aspects of Pet Rock husbandry.

There seems to be some confusion about rocks from nature and rocks formed in the urinary tract. However, when reviewing the Pet Rock literature, we were unable to find much information about the husbandry of these unique pets. Therefore, we decided to publish some fascinating research about Pet Rock nutrition.

Materials and methods

Four rocks of unknown ancestry and age were used in a 4X4 Latin square design. The average age of these rocks was unknown; their average weight was 25 grams. The rocks were fed diets containing 5 percent, 10 percent, 15 percent or 20 percent calcium. Selecting the formulation of the diets was difficult because of the absence of information based on reproducible studies of Pet Rocks. Body composition studies indicated that the rocks have a very low level of protein. Therefore, it was decided to incorporate 5 percent codfish meal collected from codfish processing plants located on the northeastern U.S. coast.

Preliminary studies indicated antimicrobials were of no detectable value, and therefore they were not used in the diet formulation. Saturated fats were also excluded in order to decrease the risk of atherosclerosis. Fiber was not used in preliminary studies of rocks of different mineral composition because there was no evidence of microbial fermentation by any type of rocks.

The original plan was to use only mature rocks, but there was no way to determine the exact age of these rocks. A geologist examined the rocks and concluded that since they were smooth-mouthed, they likely were mature. However, age may not be an important risk or protective factor because the rate of weight gain of young rocks apparently is very similar to weight gain in mature rocks.

A preliminary 20-day study was followed by a 10-day collection period. A constant level of food (2 percent of the rock's body weight per day) was provided. The rocks were fed at 12-hour intervals, and the food intake, fecal weight, and urine production were recorded every 24 hours.

The rocks were placed in metabolism cages to facilitate separate collections of feces and urine. Distilled water was placed in shallow dishes and offered ad libitum. Trace mineral blocks were offered free choice. The rocks were weighed weekly. Nitrogen content was estimated using the Kjeldahl method, and calcium concentration was measured by atomic absorption spectrophotometry.


None of the rocks consumed any of the diet during the study period. As a result, no feces or urine were collected. Although the absence of feces and urine increased the difficulty of measuring endogenous calcium losses, it decreased the number of analyses required. The loss of endogenous calcium was negligible.

Although none of the pet rocks consumed any of the special formulated diet, weight loss was not detected, indicating that the rocks' energy requirement for maintenance is very low. Water intake was measured daily. Pet Rocks consumed more water when the ambient temperature was high and the humidity was low.


The Pet Rock has an advantage over other pets (like dogs and cats) in terms of food consumption. Since their food consumption is negligible, more food would be available for human consumption. Further studies might reveal even more efficient food utilization.

Pet Rocks appear to have great longevity. In fact, the life span of Pet Rocks exceeds the lifespan of humans by many orders of magnitude. Therefore, studies comparing the nutrition of Pet Rocks to the nutrition of humans might lead to insights into prolonging the quantity and quality of human beings.

In the future, studies of Pet Rocks should encompass food consumption with specific reference to composition of minerals, including trace minerals. Pet Rocks appear to be very temperamental, which explains their finicky appetite. One hypothesis advanced by Clews et al. to explain this observation is that the Pet Rocks in this study appeared to have some limestone in their lineage (Clews et al. Journal of Irreproducible Results, 1978;12). Limestone was added to the diet formulation used in this study. The idea of cannibalism may have been offensive to the study rocks. Perhaps a different calcium salt added to the diet would ameliorate this problem.

Dr. Osborne, a diplomate of the American College of Veterinary Internal Medicine, is professor of medicine in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Minnesota.