Rapaire..Im a stonemason to trade, and this area of Scotland is full of grey and pink granite.
The best tool for trimming is a knapping hammer.Granite is very hard but quite workable.
The granite quarries round loch Fyne used to employ dozens of men cutting kerb stones,cobbles and dressed building stone.
Now the granite is crushed mechanically for road "mettle"
I still cut granite to build stone fireplaces for the "white settlers"
In my young days, built an alter for the RC chapel.
Cold chisles,with points are ok for carving,but the steel should be tempered by a blacksmith.Hard to find a real blacksmith these days
   Best Wishes Ake