Unmanaged switches cannot form a ring. In an unmanaged Ethernet network there can be only one path between any two ports on the network. If there is more than one path from one switch to another a broadcast message (and in some cases other messages) sent by the network will be forwarded until it completes a loop by returning on the second path. Since the switches forward all broadcasts and do not keep track of the messages they have sent, the returning message will be sent around the loop again and again. A single message circulating forever around a loop at high speed is clearly not a good thing, so no loops are allowed.
Ring switches allow to form rings based on unique proprietary protocol. This protocol is based on MAC address.
Managed switches allow you to form a ring through the RSTP feature. The Rapid Spanning Tree Protocol (RSTP) allows you to have an Ethernet network with extra connections, so if one path between two points on the network fails, another path can be used to deliver messages. If one link or switch fails, another link or switch can take over transparently to prevent unnecessary down time.