Electrical insulators, all made by Hemingray. They used to be common on power lines, telegraph lines, and telephone lines. They now fill page after page on Etsy. I learned this while trying to find a source for the holder shown below:
We've got about a dozen Hemingray insulators kicking around the house purely by accident -- I never set out to collect them, but there were a bunch at the farm and I've never bothered to jettison any. If nothing else, they're good paperweights. A number of years ago my cousin Chris mentioned seeing metal stands for them in a gift shop in Michigamme that turned them into votive candle holders with minimal hassle, so I bought one. It was the last one in the store.
So do you think I've ever stumbled across another source for those stands? Of course not. But Christmas is coming, and I do know someone who would actually appreciate a glass insulator candle holder, so I went looking on the intertubes -- and discovered that while there are about a zillion sites that sell insulators (and, wow, do they ever come in an amazing variety of colors) and different items for displaying insulators (nifty little LED pads that will light up the insulator from below, for example) something as simple as the holder shown above was nowhere to be seen. There is a guy on Etsy who makes an insulator holder from used horseshoes, but (a) it's $65, and (b) the recipient I had in mind for this particular gift is not into Western themes. Country chic, yes; cowboys, no.
I'll confess I don't get the attraction of collecting insulators. They're pretty when the sun shines through them, and there are some bizarre shapes (there are some that look winged), but when you can find Hemingrays in every flea market, junk shop, and "antique" store in the country, there doesn't seem to be much of a challenge involved. I have no idea why anyone would ever bother to order one from a website when a stroll through the nearest rural flea market would turn up dozens priced at about $2 each.
On the other hand, if and when a person can find a decent stand, they do make nice candle holders.