A stone was once whatever the trade said
In medieval England, goods were weighed with stones — literally, shaped stones used as counterweights — and each trade developed its own. Wool, the backbone of the English export economy, was traded in stones of 14 pounds: heavy enough to be worth counting, light enough for one person to lift and stack. Meat, cheese, glass and other trades used different stones, and a merchant crossing trades had to know which stone was in play. The system worked locally and failed nationally, which is why reformers spent centuries trying to fix it.
The 1830s fixed 14 pounds — and killed the rest
The imperial reforms of the 1820s and 1830s standardised the pound itself and, with it, the stone: the legal stone became 14 imperial pounds, exactly 6.35029 kilograms in modern terms. The other stones — the 8-pound butcher's stone, the heavier cheese stones — lost their legal standing and faded from use. What survived was the wool stone, because wool had been the trade that mattered most and its stone was already the most widely recognised.
Why 14, and not 10 or 16?
Fourteen divides neatly in the ways a pre-decimal trader needed: half a stone is 7 pounds, a quarter stone is 3½ pounds, and two stones make 28 pounds — a quarter of the old hundredweight of 112 pounds. It sat in a practical middle: a 10-pound stone would have meant more counting for a wool clip, a 16-pound stone more lifting. The number survived metrication for the same reason it survived standardisation — it was already embedded in how people talked about their own weight, and nobody with the power to change it saw a reason to.
To convert your own weight, use the stone to kg converter, and for the full conversion table see what a stone is in kg.