gloves is countable. I have two gloves. gloves can be used in singular. Ie. I have my right glove in my car. jeans is countable, although it can't be used in singular, for they change the meaning. Ie. I have three dark blue jeans inside my cupboard. jean means the fabric. Ie.I have a jean jacket. Have fun learning languages, and take care. Cheers

1. Yes we've got black ones 2. The black ones. You have to include "ones" in this context, otherwise it doesn't make sense.

Maybe they always come in pairs so you always have two, normally. My guess is the jeans (trousers) were originally considered as two pieces of garments like two socks without the heel, not sewn together. This was probably long ago, but language sticks and some things turn into expressions. Compare with the computer symbol for saving a file, it's a floppy disk! Ways change, but expressions stick. It'll be interesting to hear the real explanation now, this was just my guess.