You might try looking for sandblasting sand. It is available in different sizes and is usually composed of silica quartz, not necessarily granite-- but I think granite has a high quartz content. I have used this growing cacti & succulents from seed with good results.

Well, I thought someone with experience would join in, but . . . I am not sure limestone grit is detrimental at all, as long as it is not the crumbly, break apart in a few years kind. If they are using in for chic grit, it can't be that. And of course, if you are growing the minority of alpines that do not prefer alkaline soil, limestone grit wouldn't be good either.

Myself, I have wished I could get a hold of limestone grits. They blend so much better with plant materials, in my opinion. Granite is all that is available here. And what to use in place granite or limestone grit? Any inert matter without moisture holding capacity, and with angular surfaces. Some parts of the country have what looks like marble grit.

Even though the thread didn't start out that way,the link below is one that turned out to be a good discussion about seeding and soils. Just skip to the Feb 3rd postings where the seeding discussion begins.

Here is a link that might be useful: Anyone a Nargs seed exchange donor?

Just an aside...if one is trying to avoid limestone (CaCO3) grit, then logically, one should avoid marble as well...and dolomite, too.
Marble is also calcium carbonate; it's limestone that has undergone heating and pressure via geological processes (metamorphism), resulting in structural changes to the crystals. Dolomite is calcium carbonate with magnesium [CaMg(CO3)2]. Natural weak acids and normal breakdown will cause all 3 to leach calcium into the soil.

Good point, ABgardeneer. Still, marble doesn't breakdown nearly as quickly as any limestone. Certainly vinegar would etch marble, but not as "violently" as limestone. More speculation than fact here, but I doubt it would have much impact on soil pH. Would it?

Marble has the same chemical composition as limestone - in other words, it is limestone. Pure marble (CaCO3) has the same hardness as calcite (CaCO3)- both very soft in the relative scheme of rock hardness. Impurities (other minerals) in marble may make it slightly harder though - though, in any case, it is has low relative hardness, again in the whole scheme of rock hardness. Pure marble has just as rapid a reaction with acid as does limestone in my experience - although, again, if impurities increased to a very large amount, I suppose it may slow the reaction, don't know. The only real difference (if I remember my classes correctly, LOL!) is that limestone will fracture along crystal edges, while marble, as a result of structural changes during metamorphism, will fracture across individual calcite crystals.
Srictly speaking, I don't think we're talking about hardness - I think the important measure in this case is solubility.
Crushed calcium carbonate rock (from whatever origin - limestone or marble) and dolomite are both used as agricultural and gardening soil amendments to raise soil pH - thus, solubility is sufficiently high to effect a change, clearly - so I would think neither would be recommended in soil mixes for alpines that are calcium-intolerant.

On a different note, the local gardening center that sells alpines sells granite grit in bags, but I may need to look into sources of larger volumes this summer too. I once attended a short course from the local rock gardening association (CRAGS), and for the various soil mixes offered, the course booklet recommends:

#9 grit or similar-sized, clean, washed, crushed rock
or Gyra rock

("#9" refers to a size classification for aggregate material, probably in Imperial units - will have to read up on it.) Offhand, I have no recollection of what the heck "Gyra" rock is (other than it's a component of a soil recipe called "Josef Halda's Mix")!

Crushed rock is a product of rock quarries and aggregate quarries, so it seems that industrial sources might have to be found for larger quantities.
From googling around on the 'net, it seems that Quebec, being located on the Precambrian shield, has lots of quarries for igneous and metamorphic rock, so I would guess there must be industrial sources of graded crushed rock... Hmmm, not near as much fun to visit as garden centers, LOL!

Hmmm. . . who would have thought I would get so interested in a rock? Did a bunch of surfing, got a few answers and more questions.

Marble and limestone are chemically the same (CaCO3), but obviously "put together" differently: limestone is more porous. Having more surface area per weight or volume, limestone would also have more reactive sites per same too.

Repeated everywhere is the claim that calcium carbonate is a buffering agent that brings an acidic environment to neutral pH, but not above into alkaline. But searching for the pH of calcium carbonate in saturated solution, it is 9.3-9.8. So how that claim can be true, eludes me.

Whereas there are a few instances where pure (or almost pure) limestone as soil was recommend for a few specific species of alpines, no where did I find any recommendation or discussion to use limestone grit a large soil component in general.

So, for myself, would that I could, I'd still scarf up limestone grit for mulch over my troughs. But not as a large soil component. Trough pH can be manipulated if need be.

Rick

The introductory alpine gardening manual from CRAGS gives the following as a "good standard compost mixture" for alpines:
1 part loam or garden soil (without clay)
1 part peat moss
2 parts #9 grit or similar-sized clean, washed, crushed rock, free of clay, e.g. silica-cemented sandstone or crushed quartzite or limestone.

So, there's a recommendation that involves limestone, anyway.
So, according to local experience (and, it sounds like, your own experience), there is likely no problem in using limestone grit... except, logically, for plants that are intolerant of lime...which is, however, what the original poster was inquiring about, LOL! Whoa, full circle...

Quoting here from the NARGS 'Rock Garden Design & Construction' book:

"You are fortunate if sandstone, limestone, dolomite or marble is available locally in any form. Calcareous chippings are a premier rock garden soil additive. Calcium-rich, or limey, soils are home to many of the finest rock garden plants, including many saxifrages. In this context, lime refers to limestone (calcium carbonate) and dolomite (calcium-magnesium carbonate), which are both non-caustic even when hydrated."...

"In general, transported rock materials do not significantly affect the pH of the soil. ... The materials found in most rocks are very stable and do not break down enough to become available to herbaceous plants within their lifetimes."

There is an entire section in the book devoted to the topic of soils & rocks. If you have access to the book it makes for very informative reading.

Another NARGS publication (A Rock Garden Handbook for Beginners) says, "If a section of the garden is to be devoted to plants that need alkaline soil, crushed limestone or limestone chips may be included in the topsoil mix". (This is from page 5, in Rock and Alpine Gardens by T. H. Everett.)
Also on the same page, he says, "Hard sandstones, and not excessively soluble limestones are very satisfactory (for use in rock gardens). But remember, limestones, and waterworn limestone is one of the most beautiful rocks, are distasteful or unacceptable to such acid soil plants as heaths, heathers, and rhododendrons (sic). (Bolding added for emphasis.)
On page 23 within the same article, "Topdressing is next. Prepare a porous mix of topsoil, peat moss and grit or coarse sand as a base and modify it as needed for particular areas of the garden devoted to plants with special needs by adding additional peat moss for acid soil plants, crushed limestone or agricultural lime for lovers of alkaline soil..."

So, I agree - large, stable rocks may be unlikely to affect soil properties to a noticeable, or even perhaps to a measureable, degree. And, given that (according to my reading), most alpines favor alkaline soil or are relatively indifferent to pH, what would it matter anyway?
However, the finer the material (e.g. crushed rock), the larger its surface area and the more accelerated its (albeit, molecular) breakdown will be. So, to me, it would simply defy logic and a rudimentary understanding of chemistry and chemical processes to assume that such soil components or top dressings will not affect the pH of the soil, presumably to the detriment of calcifuge species. So, to me, it would not make sense to intentionally include crushed limestone in soil mixes or as a top dressing for calcifuge species. (In other words, I'm with T. H. Everett, above, LOL!)

The Rockies here are composed primarily of very thick successions of marine deposits, and the greatest proportion of alpine and subalpine outcrop is likely limestone, along with some dolomite. Even a lot of the shale outcrops are calcareous (i.e. formed from non-marine fines but with calcium carbonate cement). (The small outcrops of Gog Quartzite would be one of the exceptions that come to mind.) So I'm guessing that calcifuge plants might be small in number here. Can anyone comment more fully?

O.K., it's just about time for me to shut up. I had thought "calcifuge" meant lime-loving, not lime-hating. (No wonder!) But I'm cool now.

Rick

Oh, sorry! Here I thought you were supporting that the presence or absence of lime in soil really would make no difference even to calcium-hating plants....which I couldn't understand, since it seemed you were also saying that "trough pH can be manipulated if need be" (...but how if not by changing the concentration of acidic or alkaline components?) Okay, I think we understand each other then, LOL!
Whew, there is nothing in the world more difficult than communication, LOL!

I don't know why YOU should be sorry. I guess it is just the thing people say, and I do appreciate it.

Stupid Rick sure prompted an outflow of knowledge from you. I, along with everyone else reading this thread, am grateful. I don't know how you get off portraying yourself as "knowing very little about rock gardening". No maatter. We all know better.

Rick

Ummm, thanks for the compliments - and don't be so darn hard on yourself!!! Not meaning to do one of those revolting false modesty acts, but I would just point out that most of what I posted, other than direct quotes from NARGS publications, was actually about rocks, not about rock gardening.

I'm expected to have a bit more than a passing familiarity with rocks.... (Psst, don't tell anybody, but I'm actually a geologist, LOL!)