The only reason pyrite ore prices were so high in the first place is people were stockpiling it in anticipation of being able to prospect epic gems from it. Ghostcrawler specifically mentioned they expect it will take an entire patch cycle for one to replace all his gems with epic gems. It wouldn't take very long at all if epic gems could be mass produced by prospecting pyrite. That means it's virtually certain we won't be able to prospect epic gems from pyrite unless of course blizzard changes their mind.
The demand for and price of pyrite has been held artificially high for a long time by stockpilers hoping to make a killing on epic gems. Those stockpilers no longer have a reason to continue overpaying for it. The price of pyrite is already down a fair bit and will drop further as more stockpilers and farmers hear the news. The price of pyrite
Things will snowball from there causing a ripple effect in other markets. Some sellers will panic and sell too cheaply. Some pyrite will get bought out by people like me who like to play the materials markets contrary to what everyone else is doing (buy when everyone else is selling and sell when they're buying). Some pyrite will get prospected which will raise the supply of rare gems causing a drop in rare gem prices, which in turn will probably drag elementium and obsidium prices down too. Truegold, eternal belt buckles and many other things will follow like a row of dominoes falling down.
Eventually it'll hit rock bottom, the oversupply will peter out, prices will rebound and stabilize at new levels. In my experience it seems the faster and harder a market crashes the sooner and more robustly it rebounds so here's hoping it's hard, fast, over and done with quickly and with as little pain as possible.
Of course it's still always possible blizzard will change their mind yet, or they could even be playing mind games with us to surprise us on patch day but I'm not betting on either.