In the cells of most plants, crystals consisting of calcium oxalate are found which are formed by a process called biomineralisation. It is thought that these crystals act as a storage reservoir for calcium and also provide for the storage of toxic waste (detoxification). The crystals come in a variety of shapes and some are characteristic of certain plant groups.
The reason for writing something about aphids was a Willow Tree in our garden that was infested with these very small insects. Aphids mainly reside on young leaves and stems. With the naked eye there is not much more to see than small, slowly moving oval shapes that are a few mm in size. All the more reason to have a look at them under magnification.