The spell Creation (PHB, p. 229) states:
You can also use this spell to create mineral objects such as stone, crystal, or metal. The object created must be no larger than a 5-foot cube, and the object must be of a form and material that you have seen before.
The duration depends on the object’s material. [...]
Precious metals — 1 hour
Gold (which could be clearly counted as a "precious metal") has a density of 19.32 g/cm³. A five-foot cube of gold weighs roughly 62 tons.
Note that the spell does not limit whether the material has to be created on the floor or in the air. It doesn't declare any specific physic rules, either, so we can - and have to - assume that the physics apply as if it were normal materials of their respective kind.
If someone were to drop a 62-ton 5-foot-cube on top of you, you would be obliterated. Using a 9th-level spell slot, you could even create a 25-foot cube, if you so desired.
Are there any rules that would indicate how to rule / handle a situation like this?
How can I balance this spell if it can create an object that will instantly kill most creatures below it?
