Why does my gold look like this?

[sar]

I did a small batch. Have been doing small samples to see how I do. I dissolved it doing nitric increments. Precipitated it with SMB, rinsed it and melted it. Why does it look like this?

After testing this, it is only passing as 18k. I'm curious as to why. What I started with was mainly gold and a small amount of nickel. I put 700mL HCL and increments of nitric. I didn't dissolve all of it because the day is fading quick so I filtered the solution, rinsed it and melted it. Why is it not more pure? Did I do something wrong?

As you said it's because of nickel. Nickel has bad effect on the luster of gold. That's why all standard codes for 999.9 gold have a very low level for nickel impurity(≤3 ppm).
You can melt the button under an oxidizing flame for some time, remove oxides by borax several times till it shines like sun. This will remove the layer but nickel won't reach that limit (3 ppm) by this method.
If you want to get a 999.9 gold, rerefine the gold button. Don't rinse with water after filtering gold precipitate.
Boil with hot HCl two times and set the HCl aside for future use. Because it has dissolved some gold too.
After that, rinse the precipitate with hot water.
You get ride of nickel now. you can melt the nice gold powder.
By the way How did you assay the gold button that you find it's 18K? I doubt , It's more like that you use Xray .
Is that right?

 

[Geo]

It looked like it was frosted with borax. Did you sprinkle borax on it as it was cooling?