Modern day legume trees are common in South America. Resin from either of these types of trees, when placed in the proper conditions, turns into amber. This 'young' resin is called copal. Copal is always much younger than amber and has some characteristics which distinguish it from real amber. For instance, copal is generally less dense than amber.
Density is measured in something called specific gravity.
Copal looks similar, but has a lower specific gravity of 1.03 to 1.08. A specific gravity of above 1.0 will cause the object to sink in fresh water. While amber and copal will both sink in regular water, salt water has a higher density. Amber and copal will both float in salt water. In this solution, both amber and copal should float.