Gem

Gems are the fundamental towers in the maze, divided into 8 basic and 13 special combination gems with 7 different damage type colors.

To obtain gems, you must pick them .

The basic gems are  Amethyst, Aquamarine , Diamond , Emerald , Opal , Ruby , Sapphire and Topaz

Special gem combinations
The special gems available are Black Opal , Blood Stone, Dark Emerald ,  Gold , Jade , Malachite , Pink Diamond , Red Crystal , Silver , Star Ruby , Tourmaline , Uranium , Yellow Sapphire.

When playing survival mode, your first turn always grants either malachite or silver. Of these, malachite is generally preferred due getting 87,5% greater multiplier against waves with Amethyst weakness (all 40+ waves).

Quick visual guide to making them can be found at http://www.gemtowerdefense.com/ when you scroll down.