A normal pH in cats is 7.24 to 7.40 (http://www.thepetcenter.com/exa/nv.html
and scroll down to the table "Normal Ranges for a Laboratory's Blood Chemistry Values").
I don't know that a high pH can lead to calcium oxalate crystals - it can promote Struvite crystals. Struvite crystals can often be corrected by lowering the pH - this is done by feeding food that helps acidify the urine, such as Purina's "Urinary Health Tract" forumla. I believe Royal Canin has a urinary health formula as well, though I've never seen it in any stores around here. I'm sure the prescription Science Diet your vet has prescribed addresses this issue. There are also cranberry supplements that can be included in your cat's diet.
Crystals are a problem usually in male cats. They are a potential danger in that the construction of the male cat urethra is very long and thin - which, if a cat has crystals, means that a male cat can become blocked very easily. This prevents the cat from peeing, and his urine will then back up in his system and into his kidneys, causing toxicity that will kill. It takes about 24 hours or so for this to happen, and there are lots of signs that it is happening.
Litterbox hopping is the first sign. With crystals, the bladder becomes irritated. This makes the cat feel like he has to go to the bathroom. If he's blocked, he will continue trying to go - and either little drops or nothing will come out. If you notice your cat litterbox hopping, get him to a vet.
We were feeding our cats sensitive stomach / urinary health formula diet because one of our cats has a tendency towards colitis and struvite crystals. The struvite crystals, if caught early enough, can be dissolved with an acidifier called "Albeta." We keep some on hand for him.
However, this diet led our other two male cats to have calcium oxalate crystals. This is caused by high magnesium levels - not so much by high calcium levels. It is also caused by the oxalates - which are found in green vegetables. We had to stop growing and feeding them grass, which they love.
Calcium oxalate crystals (and Struvite crystals) can be peed out by the cat - unless they start building up too quickly. Then in a male they most likely cause a blockage. Sometimes it's only partial, and it can be flushed out by the administration of subcutaneous fluids, causing excess hydration and lots of peeing. Sometimes this, however, simply leads to all the crystals clumping together and causing a blockage. It doesn't necessarily require surgery.
Lazlo became blocked with calcium oxalate crystals and had to be cathetrized. He was given anaesthetic, and they put a catheter up his urethra, essentially expanding the tract so the crystals can then be flushed out. The cat usually has to spend the night in the hospital before they let him go home when this happens.
For Tuxedo, he was litterbox hopping, but every time we took him to the vet he had no pee in his bladder at all. For him, it turns out the calcium oxalate crystals had become lodged in the wall of his bladder some how. This caused irritation which made him feel like he had to go to the bathroom - but because he wasn't blocked at all, he'd pee - and then because he'd keep trying to pee but there was nothing in his bladder, nothing came out - but there was nothing for the vet to test because there wasn't ever pee in his bladder!
But the litter box hopping meant there was something wrong. The vet's ultrasound machine didn't catch it - but a higher resolution ultrasound machine used by the mobile lab our vet uses did see it. There was a clump of oxalate crystals on his bladder wall (they couldn't tell they were oxalate).
The only way to remove those was with surgery. The vet had to cut Tuxie's bladder open to remove them. He had stitches, etc. It was only upon removing them and sending them to a lab to be tested that they could tell they were calcium oxalate.
Our cats are now all on a prescription diet - Science Diet X/D. Our vet feels this is the best way to prevent calcium oxalate crystals. It doesn't seem to be causing the creation of Struvite crystals in our kitty that has that problem.
We let them free feed on dry food as well as giving them wet food once a day.
What will help prevent crystal production - or what will help at least prevent blockages if there are some crystals present - is water. We mix water in with every wet meal so what they eat is pretty runny. The only food we now give them as treats are chicken, beef or turkey baby food. We mix water in with this, too.
We also put out lots of extra water bowls. We had one fountain on each floor of the house. We put out water bowls that I clean every day - at least one for each room in the house, and two in the larger rooms. Their water intake has definitely gone up since we've made lots more water available to them. This helps keep flushing their systems.
But right now, I'd give the prescription diet the vet recommended. I'd call the vet or schedule an appointment to talk about diet alternatives to bring down or keep down the pH levels. Talk about the urinary tract health formulas that are out there, and potentially a supplement like cranberry stuff for cats. And I'd put out lots of water.
If you're worried about calcium oxalate crystals, don't feed your cat grass, and avoid foods that have lots of veges.