Hi, I'm new to the group and now that I have found you I think I will stick around for a while! I know a decent amount on costume jewelry, but am always looking to learn more!
My question for today, I found a piece at an estate sale today. It is probably from the late 1800's early 1900's dated by the unique lever C clasp. I have had a pin like this before and I think they are made out of old buttons. I have tested it with 10k 14k and 18k acids, and they are kind of throwing me for a loop. (I believe I have rubbed hard enough to go through if there was any plating.) 
I have included pics of the stone, and the pin front and back... if you want more pics, let me know!
Thank you so much in advance for any info you can give me!
On the clasp, I found this that I was using to date the pin
It is the very similar as the clasp on the left in the center row that states ca. 1890-1920's. And it is an exact match to the one in fig 13 on this page. so right around the end of the 1800's to early 1900's i think would be a fairly accurate time period.
I am testing the buttons only. The pinback is plated at best.
If the item is not gold at all, why would it then pass the 10k acid test, and looks like it almost passes 14k acid test. If it was not gold at all, those acids should eat it away completely, which it is not doing. I tested my acids on non gold, and it ate right through. I also used a piece of known 14k for comparison that my 10k and 14k acids did not eat through, but the 18k acid did, it went black immediately on the piece of known 14k.
As far as testing directly on the piece with 18k acid, I guess I don't understand what the point would be. I am not wondering if it is 18k gold, just why it would turn white like silver does with 18k acid. I wouldn't risk permanently damaging a piece of jewelry this old with no benefit from it. I know I have etched hard enough on the stone to go through any plating, and like I said it seems to pass the 10k with flying colors, and almost passes the 14k.
Are there any other metals that would pass the 10k acid test that wouldn't be gold that they would have used in the early 1900's to make buttons?
Thank you,
-This is clearly not a hand made clasp. You know the difference between a hand made clasp and machine made clasp when you see one. And You skipped an important figure from your source. I attach a picture for all of us to know the history of clasps. C clasp is the name of a clasp which doesn't have a lock but it has a 'C' catch. A simple piece of wire in shape of a 'C'.
If you want to keep calling your mechanized clasps made in 1800s, specially when you say it is not gold and 'plated at best', you can do so, but it is not correct. Jewelry history can not separate itself from industrial revolution.
-Silver is very common alloy in jewelry making. All goldsmiths use silver as alloy everyday.
- The reason that I suggested the " Notching " technique is to see if the item is rolled gold or gold plated. We want to go deeper underneath the surface. If a sample is turning white under 18K acid, most possibly, the item is not gold.
....
Anyway, don't mind me, you are doing fine by yourself 
So when did premade pinbars - available to consumers as opposed to the trade - come in? The 50s (I remember them from Brownie craft projects) or earlier? That would be a starting date for the piece-as-a-brooch; dating the buttons themselves would be a separate issue. You might look for similar buttons on button-collector sites. If you're going to sell, list it as an upcycled piece made from vintage buttons.
Nobelantique, you are correct. I was off by about a decade. This is the fig 13 that I was using to date the clasp since it matches that perfectly. I apologize. 
I was more calling it a c clasp because I have never seen one like this before, it is not your typical rollover clasp that is usually seen on brooches.
I really didn't realize that goldsmiths do use silver as an alloy when making gold pieces. To me that would explain the milky white color the 18k acid is giving on this piece. I just tested the 18k acid on a piece of silver and it did the same thing, turned milky white. So I feel confident that the top of the piece, the buttons, is at least 9-10 carat gold, and the alloy material, possibly silver, is what is turning the 18k acid white.
I was saying only the pinback is plated, not the front. I know I have gone deep enough to go through any plating. I feel it does less damage to the piece if I use the glass to rub off a corner of the piece than to make a notch, and I can see that I have gone fairly deep on the corner edge of the button. Thank you for all your wonderful help!!
