SD Chargers WR -- Mouth Full of Diamonds ... 300 Stones In Custom Grill

Tough loss last night ... but here's something that might cheer up SD Chargers WR Keenan Allen -- his brand new fancy GRILL -- complete with 14k gold and roughly 300 DIAMONDS!!

TMZ Sports spoke with Avo at A Image Jewelry -- who's made grills for stars like Chris Brown and Kid Ink -- and he says KA recently hit him up requesting a special set for the 2015 season.

The result?! A sick gold base peppered in 3 karats worth of diamonds -- and we're told the whole thing runs in the $10,000 range!

Allen is a huge fan of mouth jewelry -- and has been seen out in some of his other A Image pieces (see below) ... but the new one is next level stuff.