Hi Marigyp
I have read all sorts of things concerning lava rock and pH and have used lava rock myself for several years in various organic grow systems.
I do not have the link for the paper but there was published data on lava rock needing to be weathered before use in systems. And when the product you get isn't weathered, you get the pH climbing instead of what should be happening in systems with a large potential for beneficial bacteria like this, a slow drift down or even buffered to sit still.
Weathered - 12 months in the sun and the rain. A good wash, it seems, is not enough at times, if your product went straight from the quarry to the bag. It needs to sit outdoors and get some air etc.
Then it goes approx pH 7.0 and stays neutral.
The bacteria living in the rocks, especially as the grow progresses, should be processing nutes and releasing CO2 which is good for the plant, and part of why the pH drops in hydro systems using high biological counts if the lava rock is neutral.
Bacteria processing nutes... well, it takes nutes! This is why lava rock plants seem hungrier in new grows. It's the bacteria establishing in the rocks having a meal. Then bacterial grazers arrive and their wastes are plant food. So you've not really lost the nutes, they're redistributed.
I have read of differing colours having different pH properties but have only worked with black and red mixes and find the above info on weathering lava rock fits. It also goes by the name scoria, which could find links for you that lava rock doesn't.
It also degasses nitrous oxide and sulphides, this too is specific bacterial action taking place.
Of course, your nutrients may kill off much of the biology depending on if they are refined or organic, but thought you'd like what little info I have on lava rocks anyway.
Either way, organic or not, if the lava rock is weathered, it'll give you a lot less grief.
It should behave better each grow.