Are agatine guides suitable for today's fishing line?

For most, yes. I'd worry with wire line and can't be sure about the braided lines, but the folks at Golden Witch could probably give you a definitive answer there.

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I've used 3 or 4 agatine stripper guides and dozens of the agate stripping guides on the bamboo rods that I put together and have had no trouble with either the agatines or agates with any of my fly lines.

I would caution people that the stones in the rings can be broken if the rod is dropped and / or hit against something hard (like when putting the rod back into an aluminum rod tube). But. they look nice and work well with proper care, plus it's nice to be able to have the silk wrapping colors to match or contrast with the agate colors.