This is the same thread that I use in all of my tying because of it's intense shades of color, as you can see.
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Also I find that it's usefulness for historic patterns and fish attracting abilities is unsurpassed. 100% Filament Silk from Japan. Very thin but, because it's silk, it's strong. Doesn't FUZZ like synthetics. Fits most bobbins, but the end caps are removable to make it narrower.
Closely equivalent in size to Pearsall's Gossamer Silk (see comparison photo), so it lays well on the hook and works great for small flies. I recommend that you use either a "Bees Wax" tying wax (which also helps hold materials in place), head cement or both when tying.
Try a very thin dubbing twisted over the silk and applied sparingly to the body. When wet, the highly colorful thread shows through the fuzzy dubbing creating a translucent effect.
NOTE: Not for salt water use.
Not particularly cheap per spool compared to synthetics, but each spool contains 200 Meters (218 Yards) of beautiful, high quality, natural silk! Spooled in the US. I've used it now professionally for six full seasons, and am very happy with it. Sometimes hard to get - Advanced notice needed for large orders.
Bees wax can be found HERE