First: What a cool find! I hope you got a good bargain on the stickpin.
Second: great pictures. But even with those, who knows.... I agree with triple d: that the blob you're wondering about appears to be similar to the solder attachment of the pin (but I think the blob is on the front).
A question: isn't natural gold from the earth typically higher purity? I know different deposits vary somewhat, but I didn't think natural gold was as low as 16-18k, I thought it was more like 23k+. If true, that makes me think that someone created this "nugget" from jewelry grade gold.
As another example (but at the risk of hijacking your thread a little), I found this tie clip at an estate sale some years ago. The 2 on it is the price...$2.00! I figured for $2.00 what could I lose so I bought it.
I tested the gold (on the outer tip of the nugget opposite where it attaches to the tie clip) and it came up 23k+ using those simple acid tests.
I took it to a jeweler to ask what you were asking: is it a real nugget or not? The jeweler (a guy in the business 30+ years) said that he couldn't be sure but he did tell me that back around 100 years ago it was stylish to create and wear this sort of stuff. Jewelers were creating some of it by taking jewelry grade gold, heating it up and then drizzling the molten gold into water to create "simulated nuggets" (I guess like they make funnel cakes). He wasn't sure about my piece....and I don't think we can be sure about yours.
Sorry I can't help more.
But enjoy your stick pin - it's very cool!
- Brian
