Well dont worry... i learned this the hard way as well many moons ago.
But i will tell the story of the guy i know that wiped a coin off once...
It went from 1500 to 150 in a wipe.
Sand / dirt is the single WORST thing on a coin.
Nothing is worse.
Best is to have a bottled water with you (to drink as well) and when you pop a coin grab from edge only and gently pour water over it... DONT RUB or touch surface with anything.
Wash off completely of any particles.
Then wrap in a kleenex or even better a piece of cotton strip (rip an old cotton shirt up) and fold it over and stick in pocket,
Make sure no dirt / debris / sand remains.
When home rinse in sink with hard pressure really hot water... after rinse thoroughly... use a magnifying glass to look at both side to make sure all sand is off... then take small drop of dish detergent and drop on and GENTLY rub with FINGERS ONLY... and only for about 30 seconds each side max.
Then re- rinse soap OFF THOUROUGHLY... this is SUPER IMPORTANT not to leave ANY soap residue.
Minutes of rinsing and adjusting fingers on rim to different spots is key. HOT water rinse.
LAy on soft towel fold over and press dry.
Let it finish drying by AIR ONLY... once dried thoroughly... NEVER touch the coin again other than by the rim.
Once dried place in a NON PVC holder / item.
Plastic baggies are NOT good for coins... use for ONLY a very very very short term only until you get proper holder.
Store in cool DRY place.
If done promptly and correctly you will ensure its uncompromised integrity over time.
Everything and anything can effect a coins long term condition.
Handling and storage being the utmost 2.
Your coin is definitely the caliber of coin the above should apply.
AND.... THE / YOUR new lesson in this to ensure future finds are as they popped out.
Minus the damage of course