Can any one help identify this rock? Found in the UK, found in a common garden stone mix. Hard to drill but breaks with a hammer easily. I'm assuming it's fairly common but can't identify it.
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It's a gneiss: a form of metamorphic rock. Its composition is roughly granitic.
A gneiss is a foliated metamorphic rock: a rock that changed its mineralogical composition after being subjected to high pressures and temperatures (several thousands of atmospheres and several hundreds of degrees Celsius). "Foliated" means that it has minerals that like to orient themselves in a certain direction. Your black streaks are this mineral - called biotite.
The bulk of the rock is pink and grey - feldspars and quartz. Those are the minerals you find in granite, so that's why it's roughly granitic composition. I do not know if it actually was a granite before being metamorphosed, but I have a feeling that it did for reasons beyond the scope of this answer.
It looks more like a quartzite than a gneiss to me. the grains do not show the intergrowth that you would expect from a gneiss and the rock is breaking around the grains rather than through them indicating a lower grade (lower temperature/pressure) metamorphism than needed to form gneiss. The layers are likely relics of the original bedding.