I don't want to be too sedimental, but What kid doesn't love to rock! Let's let the kid in all of us get a little mad scientist about this pebble-cation.
All puns aside, when I was a kid one of my favorite things to do was collect rocks, stones, shells, crystals, and, if I was lucky, I might find a fossil. I wondered, Why are pointy quartz crystals always hexagonal in shape? How are fossils made? Why is hematite magnetic?
The answers have been around since about 1869 when Dmitri Mendeleev began arranging elements by atomic mass. This is our Periodic Table. Earlier documentation of identifications of patterns in the elements by chemists and scientists exist as well. In 1789, Antoine Lavoisier grouped elements based on their properties (gases, non-metals, metals, earth).
Take a Peek: The American Museum of Natural History has some handy reference information on reading the table and a PDF of the table itself in case you might need one (and who doesn't?!)
In class we have been looking at a variety of rocks, minerals, fossils, and crystals. Below you can see the result of a little research into what they're made of. To our surprise, most of the rocks fell into Oxygen and Silicon -> They're mostly made of quartz! Even the spotted jaspers, mustard colored agates, and black obsidian!
Some of the outliers were pyrite (FeS2, meaning it is made up of one iron molecule, Fe, and two sulphur molecules, S), sodalite (Na₈Al₆Si₆O₂₄ Cl₂), hematite (Fe3+2O3), calcite (CaCO3), amber (C10H16O), and the fossils.
So - if quartz and obsidian are the same, why do they look so differently? We learned in class that how fast or slowly an igneous rock cools also matters and that is displayed in the atomic layout.
Here is an example of what the lattice looks like in quartz and glass from the Corning Museum of Glass.
Here are some more fun facts about crystals:
They are repeating, three-dimensional arrangements of atoms, ions, or molecules.
Even DNA has a crystalline structure. Snow is another example.
Leaded "crystal" glass is not a crystal!
If you want to learn more, here's a nice TedEd video:
And a Crash Course on how rocks and minerals are formed. As they say, Igneous is Bliss!
More fun reading: Is now the golden age of paleontology? Did T-Rex have lips? Find out!