Crystalline gold nuggets, also referred to as crystallized gold nuggets, is gold in it's most natural form before it has been weathered and eroded by the elements. While placer gold nuggets have been carried along by glaciers, rivers, rain, floods, etc... and have been to some extent worn smooth, crystalline gold is gold that is found still attached to it's host rock (usually quartz), and is usually found in veins and pockets within the rock.
It is very rare to find any quantity of crystalline gold, because veins that rich in gold are extremely rare. Most gold found through hard rock mining is microscopic and is not visible to the naked eye. Occasionally a vein of gold will be found that is so rich, that it produces a small quantity of crystalline gold nuggets like the ones you see below. Due to their rarity, crystalline gold nuggets usually sell for a large premium over river worn gold nuggets.
Most of my crystalline gold nuggets come from locations where the mine has been closed or has very limited production, both of which mean there are few if any additional crystalline gold nuggets coming from that location, making the few nuggets that exist increasingly rare. This means that the gold specimens from these locations will become more and more rare and more and more valuable. As more people start to buy gold, more people will also start to collect rare gold nuggets like these. For these reasons, in addition to the outright beauty of quality gold specimens, they make a very good investment.