While you are not entirely incorrect, gems are often pieces of mineral crystals that, in cut and polished form, are used in jewelry or other adornments. Rocks of varying types may also be used for jewelry as well, however, that is not their main purpose. Rocks imply that they are just formations and pieces of the earth with no significant value placed upon them. So the distinction is made to make sure that no errors occur when delineating the two. Humans place value upon gems as they are hard to acquire and therefore give rise to demand. This rise in demand requires others to mine for them and thus gives them value. [ He will pause as he regards the emerald in gloved hands. ]

Glass is made of an entirely different substance. Often created with particles of sand that are super-heated and melded together. A significantly hot forge is needed to create glass, while gems are often born from the Earth itself due to pressure and other such elements acting upon rocks.