I was climbing at Harpur Hill quarry today and a hold broke off in my hand and dropped to the floor landing on my rope next to my belayer belaying me on top rope. Fortunately it missed my belayer who thankfully was wearing his helmet.
Anyway the rock (now at home as a trophy) weighs about 2lbs (so a kilo say) and dropped about 5m onto my rope. It has scuffed the sheath on the rope but hasn't cut it to the core. It's about 8m from the end of a 60m rope. If it was your rope would you ignore it or cut the rope down to about 50m.
My thought is i don't really know if anything has happened on the inside.
Short answer - If you're in doubt, cut it. Not worth the gamble.
Otherwise, its really hard to know just from a description. Take it to someone who works in rope access or rigging, and they could inspect it for you. Maybe its fine, but its impossible to say without handling it.
There's no conceivable way the core can have been cut without sustaining significant and visual damage to the sheath. If you only have minor scuffs the core will be completely fine and as strong as ever. From your description I'd keep using it and not give it a second thought.
The problem is that our brains don't always work logically, and if you're struggling to really accept that it's fine then you'll need to cut it for your own peace of mind. Better to use a shorter good rope you're happy with than a longer good rope you no longer trust.
The normal checks are to feel your ways along the rope to feel for soft spots and roll a loop passed the damaged part to see if the radius of the loop changes at the potential damaged part. If the rope folds sharply at the damaged part and or feels soft at the damaged part then it's certain there is core damage and you should chop it.
If you do chop it be sure to re-mark the centres (if you have centre marks) to avoid confusion if you or a friend is setting up an abseil on a single rope and better still always tie knots in the rope ends/get your second to tie in if multi pitching.
It's crazy how many accidents are reported to Rock+Ice related to abbing off/lowering people off the ends of a rope.