Chemical Safety Symbols
Whereas the square symbols above (which tend to be the most familiar ones) will be found on bottles & jars, diamond shaped symbols are used in transport (mainly as they can be slotted into holders on the backs of trucks & tankers to identify what the mess is (and how dangerous it is) in the event of a leak.
Other good sources for these are the HSE website, which is very complete and also explains the precise meaning of these symbols and where they should be used, as well as including common workplace safety signs. In the US, the NIOSH website includes full safety information for many chemicals and chemical families. Try also the HSE leaflets, which explain some of the issues relating to chemical transportation and various aspects of laboratory safety. Above all, remember that this explanation is for curiosity only - there's no guarantee that any of this is correct!