Crystals, Stones and the pH Story

At a Glance

Crystals in cat urine are not the same thing as bladder stones in cats — and neither is automatically a disease. This guide untangles crystals, stones, and the chemistry that links them.

Two minerals cause almost all the trouble: struvite and calcium oxalate. Together they account for roughly nine in ten feline uroliths, and they form under opposite conditions, which is why cat urine pH gets so much attention, and why chasing it alone fails.

The genuinely good news: a vet-prescribed urinary diet can often dissolve struvite, typically within two to five weeks. Calcium oxalate cannot be dissolved, so with oxalate, prevention is the whole game.

Concentrated urine is the common enemy. Dilution beats any clever chemistry, which is why the strongest prevention lives in the water playbook, not the supplement aisle.

The tray is your early-warning system: blood in cat urine, gritty “gravel”, straining, or a change in cat urine clumps in litter all earn a vet visit — and a male cat straining with nothing coming out is an emergency, now.

The Full Guide

First, the Cold Read: When This Is an Emergency

Pet parents often meet this subject at the worst possible moment. Here’s the line that cannot wait.

For everyone else (the cat with a crystal note on a lab report, the gritty deposit in the tray, the stone found on an X-ray), the rest of this guide is the calm version of the story.

Gravel, Crystals and Stones: Three Different Things

Pet parents often say: “The report mentions crystals in cat urine; does she have stones?” Here’s the untangling.

Crystals (crystalluria) are microscopic mineral flecks in the urine itself. They are a chemistry reading, not a diagnosis: perfectly healthy cats can show them, especially in the concentrated urine cats naturally produce.

Stones (uroliths) are what happens when that chemistry runs unopposed for long enough: crystals aggregate into hard mineral masses, from sand-grain “gravel” up to pebbles, which can sit quietly, inflame the bladder, or travel and block.

Plugs are a third thing: soft protein-and-crystal putty, and the commonest single cause of blockage in males. They belong to the emergency article, so we will not repeat them here.

The Two Minerals That Matter

Pet parents often hear the names and lose the thread: “struvite or oxalate, which is the bad one?” Both, in opposite ways.

Struvite (magnesium ammonium phosphate) tends to appear in younger to middle-aged cats. In cats it usually forms without infection, in urine that is concentrated and not acidic enough. Its saving grace: it is the dissolvable one.

Calcium oxalate tends to arrive from middle age onwards, and some breeds carry more risk: vets see it more in Persians, Himalayans and Burmese, though any cat can form it. It is the stubborn one: no diet dissolves it.

Between them these two make up roughly nine in ten feline stones, in proportions that have see-sawed for forty years — and that see-saw is the most instructive story in this whole subject.

The pH Story, and the See-Saw It Explains

Pet parents often ask about cat urine pH as if there were one perfect number. Here’s why the truth is a balancing act.

Struvite is genuinely pH-sensitive: it forms far more readily in alkaline urine, and making urine mildly acidic helps prevent and dissolve it. Calcium oxalate is only weakly pH-sensitive — and pushing urine strongly acidic can raise its risk rather than lower it. In a large dietary study, cats on the most acidifying diets carried roughly three times the calcium oxalate risk of cats whose urine sat mildly acidic.

Which explains the see-saw. In the 1980s struvite dominated, and the industry answered with heavily acidifying, magnesium-restricted foods. Struvite fell — and calcium oxalate rose to take its place. Magnesium spent a decade as the villain, yet the same dietary research associated higher-moisture, less acidifying diets with lower oxalate risk. The lesson is not that pH is unimportant; it is that chasing one number or one mineral swaps one stone for another.

How Stones Announce Themselves, and How They Are Found

Pet parents often search: “cat urine clumps in litter”, “blood in cat urine”, “gravel in the litter tray”. Here’s what those observations are worth.

The tray usually speaks first. Blood in cat urine, straining with only small amounts to show for it, many small clumps where there used to be a few large ones. Or the quieter versions: urinating outside the tray, gritty “gravel” stuck to paws or tray liner, a cat who licks under her tail after every visit. Each is a reason for a prompt appointment, within days rather than weeks. Some stones say nothing at all and are found by accident.

At the clinic, diagnosis is refreshingly concrete: a fresh urine sample read promptly, and imaging. Both major stone types show up well on X-ray, and ultrasound fills in the rest. Your vet can usually tell you the same day whether stones are present, roughly how large, and (from the urine picture and the stone’s appearance) which mineral is the likely culprit.

Treatment, Honestly: Dissolve, Remove, or Watch

Pet parents often assume stones mean surgery. Often they don’t. Here’s the honest decision tree.

If it is struvite, food is the treatment. Specialist consensus is unambiguous: struvite stones should generally be dissolved rather than cut out. A prescribed urinary diet can dissolve struvite completely, typically within two to five weeks, with progress checked on repeat imaging — a genuinely satisfying piece of medicine in which dinner does the surgeon’s job.

If it is calcium oxalate, it must come out — when it needs to. No diet dissolves it. When removal is warranted, current guidance favours the least invasive route available: flushing small stones out, retrieving them with instruments, or breaking them up, with open surgery reserved for cases that need it.

And some stones are watched, not treated. A quiet stone that is not causing signs and is unlikely to block does not automatically require an operation: monitoring is a legitimate plan. That decision balances size, position, mineral and your cat’s sex, and it belongs in an unhurried conversation with your vet.

One honest expectation to set: stone-formers tend to re-form, calcium oxalate especially, over months to years. That is not failure — it is why the follow-up rhythm of urine checks and occasional imaging matters as much as the first fix, and why prevention is the real long game.

Prevention: Water First, Then the Right Diet

Pet parents often ask: “Which food stops stones?” Water is the food that stops stones. Then comes the label.

Dilution is the master lever. Every stone type forms more readily in concentrated urine, and no mineral trick outperforms simply lowering the concentration. More moisture in means more, paler urine out, and the practical methods, from wet food to the tricks that make water interesting, are exactly what How to Actually Get More Water Into Your Cat at /articles/how-to-get-cat-to-drink-more-water covers.

The right therapeutic diet, matched to the right stone. A vet-prescribed urinary diet is genuinely effective, but struvite and oxalate pull in different directions, so the diet must be matched to your cat’s actual mineral, history and urine picture. This is precisely where your vet earns their keep; a diet chosen off a shelf to a guess can feed the opposite stone.

Keep weight in the healthy range, keep the tray appealing so she empties her bladder often, and keep the routine checks: a urine sample once or twice a year is a small habit that catches the chemistry drifting long before a stone forms.

From Your Words to the Vet’s Words

Describing what you have noticed in the clinic’s language helps your vet get there faster:

Where to Go Next

Pet parents often ask: “So what do I actually do?” Here’s the rest of the picture.

Understand why cats run concentrated in the first place. Start with Water Is the Whole Story here.

Do something about it. The intervention playbook is How to Actually Get More Water Into Your Cat here.

Know the emergency cold. If a cat is straining and producing nothing, read When a Cat Can’t Pee here — and go to a vet now.

And if you are weighing up what you are seeing in the tray, our screening tool will walk you through it in a few minutes and give you something concrete to take to your vet. It is a screening aid, not a diagnosis, and if your cat cannot pass urine, skip the tool and go straight to the clinic.

Sources Cited in This Article

1. Lulich, J.P., Berent, A.C., Adams, L.G., Westropp, J.L., Bartges, J.W., Osborne, C.A. (2016). “ACVIM Small Animal Consensus Recommendations on the Treatment and Prevention of Uroliths in Dogs and Cats.” Journal of Veterinary Internal Medicine, 30(5), 1564–1574.
2. Lulich, J.P., Kruger, J.M., MacLeay, J.M., et al. (2013). “Efficacy of two commercially available, low-magnesium, urine-acidifying dry foods for the dissolution of struvite uroliths in cats.” Journal of the American Veterinary Medical Association, 243(8), 1147–1153.
3. Lekcharoensuk, C., Osborne, C.A., Lulich, J.P., et al. (2001). “Association between dietary factors and calcium oxalate and magnesium ammonium phosphate urolithiasis in cats.” Journal of the American Veterinary Medical Association, 219(9), 1228–1237.
4. Kopecny, L., Palm, C.A., Segev, G., Larsen, J.A., Westropp, J.L. (2021). “Urolithiasis in cats: Evaluation of trends in urolith composition and risk factors (2005–2018).” Journal of Veterinary Internal Medicine, 35(3), 1397–1405.
5. Albasan, H., Osborne, C.A., Lulich, J.P., et al. (2009). “Rate and frequency of recurrence of uroliths after an initial ammonium urate, calcium oxalate, or struvite urolith in cats.” Journal of the American Veterinary Medical Association, 235(12), 1450–1455.
6. Taylor, S., Boysen, S., Buffington, T., et al. (2025). “iCatCare consensus guidelines on the diagnosis and management of lower urinary tract diseases in cats.” Journal of Feline Medicine and Surgery, 27(2).
7. Buckley, C.M.F., Hawthorne, A., Colyer, A., Stevenson, A.E. (2011). “Effect of dietary water intake on urinary output, specific gravity and relative supersaturation for calcium oxalate and struvite in the cat.” British Journal of Nutrition, 106(S1), S128–S130.
8. Merck Veterinary Manual. “Urolithiasis in Small Animals.” merckvetmanual.com
9. International Cat Care. “Urolithiasis (urinary stones) in cats.” icatcare.org
10. VCA Animal Hospitals. “Bladder Stones in Cats.” vcahospitals.com