Wow, there is so much negativity about a fun, educational item. No, you do not have instant gratification with rock tumbling--you are simulating in 4-8 weeks what nature takes tens of thousands of years to do.
This isn't an item for kids that can't wait to see results. I can see a level of impatience with many children. It is noisy at first. Also, if you are buying the "toy" kit you may not get the results you want.
We live in a state with many gem mines and love to do the sluices along the road side. We have a nice double barrel Thumler's Tumbler and love to do rocks. It takes time. There are multiple stages of grit tumbling, starting with course grit and working to fine, polishing beads. Each stage takes 10 days to 2 weeks, occasionally a stage needs to be repeated. You must put rocks of like hardness in the barrel in order to get good results. We have tumbled emerald and aquamarine and gotten some beautiful polished specimens. It took around 3 months to do these since they are so hard. We also tried to polish a round of sapphires and rubies, but got nothing since the stones were too hard to tumble with the grit we had.
We keep the tumbler in the garage. It makes some noise--the barrels are rubber and it muffles the sound. The earlier stages where the rocks have jagged edges are louder. The tumbler is designed to run weeks at a time so it isn't a fire hazard--you don't unplug your refrigerator do you? We don't see any changes in our electric bill either.
I do not recommend the "toy" models like you purchased. Because so many people will get tumblers and lose interest, your best bet is to look for the Tumler's Tumblers on ebay and purchase the grit packs and stones separately. The polished stones can make some lovely wire-wrapped jewelry, but you can purchase tumbled stones if your child can't wait. I can see this being an item that just won't hold a young child's attention for a long time since it takes weeks/months for results. Honestly, if she hasn't asked for the tumbler, I would return the toy model, but look for a used better tumbler if she expresses an interest. Good luck. (And I find rocks in my husband's pockets all the time.
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http://www.thumlerstumbler.com/
