Quartz!

Here is where I found this image: https://www.charmsoflight.com/Images/Gemstones/Quartz%20crystal.jpg

The link shows an image of Quartz. I think it would create interest for the students because it looks very pretty and interesting. It would be a good way to have the figure out what it is and how they know what it is. Showing them different minerals or rocks would be a great way to have them start identifying properties of minerals such as luster, color, etc. By creating their own classifications first, it would lead them to understand better why we have the classifications that we have today.