Like Geology, the History of Curling is written in the rocks.
Some sources believe that Curling was developed from Quoiting: a very ancient game where objects were thrown through the air at a target. The modern game of Quoits throws metal rings at a metal spike, and many variations exist.
The Romans threw horseshoes (bent into a ring to resemble a discus) for distance, until someone drove a stake into the ground and everyone started throwing for accuracy.
A simple game.
Some accounts suggest that the earliest Curling stones weighed as little as six pounds, and were thrown through the air, not across the ice.
The Stirling stone, right, is generally believed to be the oldest known Curling stone. It was found when an old pond was drained in Dunblane, Scotland.
These early stones ("Kuting" stones) did not have handles, were not round, and just have a niche for fingers.
Some suggest that this stone is older than the date marked, 1511.
Early accounts suggest that these stones were thrown for distance, not accuracy, and Curlers used "crampits" to get a firm grip on the ice, but no physical artifacts, and no contemporary written accounts survive.
Stones with handles emerged in the 1700's, but they bear little resemblence to modern stones.
This one is dated 1700, and the written record is much better.
Stones were not shaped in any way, except to make holes to attach the iron handles.
The stones had names, too.
One account describes "Whirlie": a stone of oblong triangular shape which was nearly impossible to remove once placed on the tee. They say that any attempt to take it out simply set the stone spinning.
Weights varied greatly, with stones like "The Soo" weighing 79 pounds, and "Robbie Dow" weighed 34 pounds.
The Muthill Curling Club in Scotland has a stone named "The Bible" which is square.
Anything from 9 inches to more than 18 inches seems to have been normal.
The heaviest of these old stones is 117 pounds.
Thankfully, the length of a "rink" used at that time was less than a quarter of todays' standard curling sheet.
Curling stones became round in the early 1800s, but weight still varied a lot.
Round stones don't have the individuality of "Whirlie" and naming them fell out of style.
By the mid-1800's Curling Stones began to take their familiar form, and manufacturers like Kays of Scotland were producing stones that were rounded, polished and of a standard shape and weight.
(Wondering about the title? Igneous rock forms when magma cools. If it is extruded into the air, you get basalts and pumice. Magma that cools more slowly, underground is called Plutonic rock. Granite forms in this manner.)