In a previous post,
Heather provided a quality comment needing to have a reply including a little more than words.
What Stone Is That?
Heather, one of the best things about sharing learning through blogging is the way we can exchange ideas so easily no matter where or how far we are apart. Your comment was an example where we both learn from each other. Reading what you shared, I realised I would need to share extra graphics to give you some answers.
Your suspicion the rock might have been microline had me examine the microcline more closely, something you can't do easily from a picture. When I looked closely, the way the crystals had formed suggests Keira's rock wouldn't be microline. Her rock looks too smooth to match the microcline sample I have. Your observation was good even if it may not be correct. My suggestions might also be wrong. It's by presenting different ideas and solutions science gets understanding. Your comment meant I had the chance to think about and research other ideas. 🙂
How Quartz/chalcedony/agate forms...
A concentric, banded, fibrous variety formed by precipitation from watery solutions in rounded cavities in lava rocks (geodes), sometimes with beautiful clusters of rock crystals or amethyst at the centre.
It was a little unfair of me to leave just the above without explaining it so I was impressed you asked for a simplified explanation. It meant you were really trying to understand. Here is an explanation in other words...
A concentric, banded, fibrous - is about the fine layers you see in this magnified part of your class photo. Fine layer after fine layer had built built up making bands in the sample.
formed by precipitation from watery solutions in rounded cavities in lava rocks (geodes) -
Water can absorb many types of minerals, some slowly and some quickly. When the water evaporates, the minerals can be left behind. In some examples, crystals can grow. Here's something you can try. Take salt and dissolve it in warm water or take some sea water, the saltier the better. Put the water in a flat, clear dish (glass is better so you can see under the result) and leave it in a sunny, warm place. Don't move or stir it as this can effect the results.
Over time, the water will evaporate and you will see salt crystals forming. Once the water has all evaporated, you can examine the crystals. Use a magnifying glass to see the finer crystals. Crystals are fascinating. If you see a larger crystal, you might see it has a certain number of sides. How many can you see?
Crystals are growing rocks. We don't consider them alive but they do grow as long as the "watery solution" is still able to provide raw materials. Here are some crystal photos I have taken to show you.
You can see quartz crystal has six sides.
Citrine Quartz Crystal
The different colours in quartz can depend on other minerals dissolved in the watery solutions.
Amethyst (quartz) Crystals
Blue Calcite crystal
Sulphur (Sulfur) Crystals
This sulphur sample has been inside a sealed container for over 30 years. With permission, it was collected from a fumerole (where gases and steam escape from volcanic areas) in New Zealand. Sulphur (and salt) crystals break down in water so they would have become powdery instead of shiny if not in an airtight container.
Iron Pyrite (fool's gold) crystal
Keira's Rock - How did it get there?
Rocks can sometimes be found a distance from where they formed. They can be moved by erosion (the wearing away of rocks by wind and water) or perhaps even movement in the Earth. Let's look at some samples you might recognise...
Gold can often be found associated with quartz rock but is much rarer. Most quartz won't have any gold. Just like other minerals, small amounts of gold can be in water and other fluids (it doesn't really dissolve but can be present in very small amounts). As water evaporates, gold can be left behind in cracks in rocks. If enough gold is carried in enough time, it can build up in amount. The sample of quartz below is from a mine in Hill End, N.S.W.. This is mined gold in quartz.
If the gold bearing quartz is exposed to wind and rain, larger pieces of gold can be eroded out. This is how we can find gold nuggets in streams and rivers. The below sample is only 0.127oz or about 1/8oz (3.6g). It is only about 1.4cm across. You might even be able to see a tiny quartz crystal in it. This small nugget came from New Zealand.
Finer pieces of gold can be eroded off larger pieces or out of quartz. To retrieve this gold, panning is often used. I panned some of this in Australia and New Zealand and bought some. The amount is 0.264oz or about 1/4 oz. (7.5g)
Gold starts it's journey as precipitated (left behind when water dries) gold in rock. It can be mined or found in nuggets and flakes if eroded. Gold, as you know, is used in jewellery and is also import in electronics but my favourite use is to make gold coins. The photo below shows a 0.1oz Australian Gold Nugget coin. It has a diameter of about 1.5cm and its value for the gold content today is about $135 U.S..