Typical speciesist thinking.



The above answer is obviously totally wrong based on incorrect human-centric notions of normality.



Horses would not think in terms of carats (which are based on carob seeds), but rather in terms of carrots (Daucus Carota Sativus.) This makes the gem the approximate size of 20 "standard" carrots, which are in human terms about 60 grams or (about) 2 1/9 ounces each...



Thus, 20 times 2 1/9 oz = (about) 42 1/3 ounces, or, right at 6000 human carats... which would be a jewel of considerable size by any measure.



The dimensions/size of such a gem of a naturally faceted crystal jewel would be on the order of around a 4 inch average dimension (overall length and height), or just about the same size as your average human wiffle softball. Mind you, this is for a relatively lighter specific gravity semi-precious jewel, such as a Carnelion, a bit smaller for a Garnet, smaller still for a Sapphire, and much smaller for a Diamond.