Probably magnetite an fe containing mineral.
Why does a magnet stick to granite.
Having dinner this evening around the granite kitchen worktop bar.
Long before people invented the small magnets that stick to refrigerators or the big magnets that pick up cars at the junkyard people discovered natural magnets.
We have some useless fridge magnets shaped like giant clothespegs to clip notes reminders shopping lists to useless since they get knocked off the fridge fall to the floor and the magnet pops out of the casing.
This allows the minerals which form plenty of time to grow and results in a coarse textured rock in which individual mineral grains are easily visible.
Granite is intrusive which means that the magma was trapped deep in the crust and probably took a very long time to cool down enough to crystallize into solid rock.
Iron oxide is a fair permanent magnet ferrite magnets so if there is iron oxide in the granite it will be weakly magnetic unless the granite is laid on top of an fe containing material and your magnet is actually attracted to what is below.
Granite can be slightly magnetic.
The most magnetic and common type is a lodestone.
Especially if it is mined from india.