A Key You Run

1. Put a drop of vinegar on a fresh edge. Does it fizz?

   • Yes, it fizzes go to 2
   • No fizz at all go to 4

2. Can you scratch it with a copper coin?

   • Yes, easily — it is soft and often chalky on the fingers ChalkSoft, pale, powders on your hands. Made of shells too small to see.
   • Not with a coin go to 3

3. Look at a broken edge. Sugary sparkle, or dull and sometimes shelly?

   • Sparkles like sugar MarbleLimestone that got hot and squeezed. The shells are gone; the crystals grew.
   • Dull, maybe a fossil in it LimestoneLaid down as shell and sea-floor mud. Fossils are common in it.

4. Are there layers, bands or sheets on a broken edge?

   • Yes, layers or bands go to 5
   • No layers — it looks the same all through go to 7

5. Rub it. Does it feel gritty, like fine sandpaper?

   • Gritty, and you can see the grains SandstoneSand, buried and glued. Often you can rub a grain off with a thumbnail.
   • Smooth, and it splits into flat sheets go to 6

6. How thin do the sheets split, and does it ring when tapped?

   • Very flat, thin sheets, rings slightly SlateMud, squeezed hard. It splits along the squeeze, not along the old layers.
   • Crumbly dull layers, no ring ShaleMud that was buried but never squeezed enough to become slate.

7. Can you see separate crystals, and how big?

   • Yes — big enough to tell apart, speckled light and dark GraniteCooled slowly, deep down. Slow cooling is what gave the crystals time to grow big.
   • Too small to see, dark and heavy BasaltThe same sort of melt as granite but cooled fast at the surface, so no crystal had time.
   • Full of little holes, and very light PumiceFroth. It cooled around its own gas bubbles, and it is the one rock that floats.

Nothing fits? Write down what the tests said and look it up later.