That is an amber doorknob. And a cast bronze one too.
Brass mounted glass doorknobs come in two ways from 2 distinct time periods.
The early ones are always plainer with fewer facets to the glass. But they turn purple in the sun. Sometimes grape juice purple!! The old manganese glass bleach was being used and this is what breaks down and turns purple.
The best 'scutcheons (the backing plate covering the lock) come from this period and solid brass finely cast and worked escutcheons are actually rarer than the knobs themselves. A great pair of plates is --way-- desirable.
The later period saw more elaborate glass molding with many facets or petals. They mostly came with smaller plainer scutcheons, often nickel plated.
These were made during and after WW1, so the nation was off of manganese glass bleach, and onto selenium glass bleach.
Manganese we mostly got from Germany (hence the shortage) so during the war selenium was found to work, and we had plenty, and it didn't take so much to work, so it was cheap.
Selenium is still to this day used to bleach melted sand into crystal clear glass. (melted sand is almost never ever clear by itself)
But in the early days of use, they used too much. Too much selenium will turn straw gold in the sun, or pale pale amber.
You can leave it in the sun for another 300 years it won't change much more though.
Not like purple that gets darker every year.
PS The bronze knob is probably yellow bronze or maybe even brass.
But a very few of these were cast from red bronze.
Red bronze polished bright, is a deliriously delightful color. Eat it with a fork color! Red bronze rocks!
yours Scott
