Nothing weird about it. Did you not take chemistry in high school? Or have you not gotten that far yet?? That's exactly what one expects to see when Mercury thiocyanate decomposes!
I don't recall working with anything quite like that in my high school chem lab, to be fair. Now that we know the "what"... I would like to know the "why". I would assume the consistency of that final product is ash-like?
This chemist's party trick is called "Pharaoh's Snake" - I presume not the one-eyed trouser variety. I had to look it up to find out the reaction scheme. This is quite complicated but is explained here: http://www.chem-toddler.com/redox-reactions/pharaohs-snakes.html It appears the snaky product is carbon nitride, C3N4, in highly vesicular form (due to the gases that are liberated), a bit like coke. Lots of nasties are given off, including mercury vapour, so not one to try out on the kitchen table.
Yeah, it was the toxicity of Pharoah's Snake fireworks that led to them being replaced with Black Snake fireworks - apparently there were even deaths of children, which variously contain things such as sodium Bicarb & Sugar or nitrated Linseed Oil and napthelenes.
Here's some more weird chemical reactions. See anything that surprises you? https://www.youtube.com/watch?v=fhAZmBNrE3I
Some I don't immediately recognise, but many are old favourites. Towards the end there are also a number that are not chemical processes but physical ones, e.g the magnetic levitation and the crawling ferromagnetic liquid. One they don't have is a favourite from my 6th Form schooldays called the Cape Canaveral experiment. In this, an ignition tube is placed on its open end on a table, after being filled with a mixture of hydrogen and chlorine gases. A piece of magnesium ribbon is then lit and brought close to the tube. The tube takes off vertically in an explosive manner, illustrating the chain reaction of H2 with Cl2, which is initiated by UV light. Elegant, I always thought. If it's a sunny day one can initiate it with sunlight (need a blind to prevent it going off prematurely).
The magnetic levitation one is fun. At the end of the day it's just eddy currents making their presence known, but, I used the same principle to build a rail gun in my lab using a high current coil and a disk with a hole punched in the middle of it.
