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When is a stone not a stone? That's not the cue for a chin-stroking debate. It's actually a question that archaeologists and museum curators often have to answer.

The first week of A History of the World in 100 objects, the omnibus of whichÌýwas on Radio 4ÌýlastÌýFriday night,Ìýdid a very nice line in ancient bits of stone that are in fact deliberately-shaped tools - the very earliest human technology. But knowing your axe from your pebble is no mean feat. In fact it's very easy to mistake a naturally-shaped rock for an ancient artefact.

Jill Cook is the curator of the oldest objects at the British Museum and has spent many years studying and identifying the most ancient tools in existence. She told me what to look for.

Stone tools are made by using one stone as a hammer to knock bits called 'flakes' off another. When a toolmaker is at work, these flakes originate on an edge and are systematically removed in a sequence from one or both sides.

When the flakes are knocked off, they leave shell-shaped depressions or 'scars' on the edge being modified. These scars are narrow at the edge and expand out removing much of the natural surface.

Such modification produces artefacts that show pattern and purpose. When such tools are found together among the flakes that were detached from them and sometimes with cut-marked animal bones, we can easily recognize them as tools.

So, if it's got 'flakes', 'scars', 'pattern' and 'purpose' then it might just be the handiwork of a distant ancestor. Armed with this knowledge, have another look at the Olduvai handaxe or Clovis Spear point and you can really see what Jill means.

But how does she explain the seemingly unnaturally-shaped stones that we sometimes find.

Stones modified by rock falls, waves crashing them together on a beach, cattle trampling, or extremes of temperature, look quite different. If a stone is only nibbled around the edges, this suggests that the edges are accidentally damaged by agricultural activity.

Random scars struck individually from different parts of the surface are a sign of natural impacts, whereas round scars that do not originate at an edge are a sure sign that the stone has suffered extremes of temperatures causing thermal fractures which have a characteristic saucepan lid shape.

Look closely enough and the clues are there. But if you want a second opinion local museums or Finds Liaison Officers from the can help separate the stone from the sculpted.

Although even Jill would admit that it's a tricky business, no matter how educated the eye.

Naturally modified stones often do look like tools and can fool the most experienced archaeologists.

Nice to know you're in good company eh?