Diamonds are large, sharply defined THCA Crystals. The difference against small crystals is only size, and how long they were left to grow.
They come from a method called diamond mining: a solution is left to sit undisturbed under pressure in a sealed vessel for weeks. The slower it goes, the larger and cleaner the crystal grows. The liquid phase stays around it with the aroma concentrated in it.
Which is why they are sold as diamonds & sauce — the crystal and the liquid belong together.
Handling: a crystal is hard and slippery, so you want a paddle with a lip or a scoop. A needle will slide straight off it. And the glass needs steady heat, not a hard rip — otherwise it melts and sits on the floor.