Dring visits to Culzean in Ayrshire we usually found agates on one of the beaches, some of which are these tiny ones.  They were formed as a result of volcanic activity when gas bubbles form holes or cracks in the cooling lava. Over time water that is rich in silica seeps in depositing layers of silica eventually forming different layers of colour depending on pressure, temperature and mineral content.  The slow process can take millions of years.
Some of the other pebbles show how they look before they are broken by the action of the sea
Thanks to loisbiz for hosting the "hard"  tiny challenge.