An "aluminum crush washer" or conical seal is sometimes placed between the flare surfaces. These are mechanic band-aids to be used when you have a leaking connection and no replacement fittings. The usual problem is using a new hose fitting against an old male nipple whose sealing surface has been damaged. It's easier to drop a conical seal into the hole than to replace the damaged fitting. Going back 10-15 years no one had ever heard of them.
Conical seals getting more popular now, in my opinion, because of the increased usage of steel on steel mating surfaces. Traditionally, AN plumbing mating surfaces were aluminum to aluminum or steel to aluminum. To seal any surfaces, one or both surfaces must conform (yield) slightly under pressure to seal any microscopic gaps between the surfaces. It helps if one or both surfaces are relatively soft. The more recent switch to steel on steel requires increased nut torque to get the two hard surfaces to yield sufficiently to seal. This should not be a problem with good surfaces and proper torqued.
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