I think this is the first time my picture in the opening post has failed. Here's the pic.

That's right, Socks. I am a Licensed Geologist in Washington. In that bowl, on my phone, it's hard to tell it from pumice. Try Sock's vinegar test, if it fizzes you've got limestone.

Good idea. I placed a few bits in vinegar. There were little lines of bubbles constantly coming from specific points on the rocks. Bubbles didn't come from the whole surface of the rocks. I was able to scratch test a fingernail and penny with one of the rocks. I found this which gave me a list of common items to use for testing. I'm not sure if I have a nail made of pure iron so I skipped that one. The rocks don't scratch glass. The more I handle the bits of rock the more they fracture and break in to smaller pieces. Whatever this is it seems to have at least some calcium carbonate in it.

Years ago I bought a bag of marble rocks that was on sale but then I wasn't sure what to do with it. Just now I got a rock out of the bag, rinsed it off because it had dust all over it, and placed the rock in some vinegar. It fizzes more than the rocks in my picture in my second post. So.. could my marble rocks be a low quality marble that is still part limestone?

That's a possibility, but based on the fizz you report and how the stuff seems soft, I'd hazard a guess its limestone. Whe?re did you get the stuff? It certainaly is not marble, that stuff is fracture resistant and will not fixx if you pay it, not to mention its hard as a MF.

Rob, the small rocks in the first pic came from the Oregon coast. I don't know much more than that. One of my nephews went there, brought back the rocks, and dumped them at my house for some reason.

Below is a pic of the questionable marble that I mentioned. I bought a bag of it that was on sale at Walmart years ago. These fizz more than the small rocks from the Oregon coast in my first pic.

I went to a local canyon and collected a few rocks that look like limestone to me. They fizz about as much as the questionable marble and more than the rocks from the Oregon coast.

These rocks don't break apart by just being handled like the ones from the Oregon coast.

I've been reading that maybe I don't need to bother with making a soil mix that has limestone for my cactus. Seems like the importance of the limestone is unclear. The cacti might grow on the limestone just because other taller plants don't grow there. However, I found this article and Escobaria zilziana is on the list.

Ive used limestone gravel. Didnt cause much problems in the long run. Only lost plants there were mishandle or already ill. But ,make sure to add high acidity soil to the mix...

I learned why my marble rocks fizz. I thought maybe after being metamorphosed and becoming marble it shouldn't fizz but it does. It is still calcium carbonate.

I was looking at the list of cacti that grow in limestone I linked to earlier. I have Escobaria sneedii and Mammillaria plumosa which are on the list, too. However, I have my plants in a gritty mix that doesn't include limestone and they seem OK. I'm not sure what to call the grit the M. plumosa is in. The E. sneedii is in pure decomposed granite. Tonight I took a couple divisions from each plant. Maybe I'll try rooting them in a soil mix that includes a lot of fizzy rocks, maybe rocks like in my last picture, to see if the cacti grow differently.

Great idea to do an experiment.

We have a TON of limestone here because of the Canadian Shield. It's sold by all the garden supply companies. I haven't dared to try using it yet because of alkalinity, but please let me know how it goes Poly!

I went back to the canyon to look for more limestone in an area rumored to have limestone. I found some bits of rock that looked more natural than the rocks in my 3rd pic. I've seen and heard of a kind of slag that is sometimes used in my area that is said to discourage weed growth as it breaks down. The rocks in my 3rd pic have a somewhat uniform size, suggesting it might not be natural to the area, which made me have second thoughts about using it. Here's the last batch of rocks I found.

The size is better for use in small pots, too. Anytime I washed these rocks and handled them I was left with a white/grey residue on my hands. They fizz as much as the rocks in the 3rd pic. I put these rocks in a mix of pumice and potting soil, equal parts. I usually use collected rain water for watering. I repotted the Escobaria zilziana and put two divisions of E. sneedii in this mix. I put one Mammillaria plumosa in this mix. I had a second M. plumosa division but it was bit soft for some reason so I discarded it. It will be a while but I'll return and report if I remember.

The M. plumosa was shriveling away in the limestone mix. I just snapped it off the parent plant, not at the joint I guess. I put it in a different gritty mix, more humid environment, to try to root it. M. plumosa is fleshy cactus that is sometimes tricky for me to root divisions. I think I ordered a M. plumosa from Mesa Garden. I'll try that one in a limestone mix.

I found this at a local bulk rock seller. They sell by the cubic yard or bag.

They call it ivory stone but the person I talked with didn't know exactly what kind of rock it is. It fizzes in vinegar. I think for now I'll use my rocks collected from the hills. I guess I could use my crushed marble, too, but doing the crushing is inconvenient. The rock in the pic could work too but again I'd have to buy a bag of it and I would have to crush it.

The roots of my E. zilziana rotted. The top of the plant was plump and looked OK but I noticed it was leaning a bit and when I touched it it was a bit more floppy than it should be so I pulled it out of the pot. The roots turned to mush. I must have over watered it. I cut it in half and dusted the cut end of the top portion with a mix of rooting powder and cinnamon. Maybe I can graft a few tubercles to some Pereskiopsis, too.