I'm trying to figure out where the anniversary symbols come from (ie paper is the traditional gift for 1st anniversary, wood for 5th, etc.). I've tracked them down to a mention in Emily Post's guide in the early 1900s but also found a mention of them in an 1800s Dictionary of Phrasese and Fables... My impression is that the gifts come from various cultures based on traditions of reciprocity, but I wonder when they were standardized and what the traditional gifts in each category were at that time.

Does anyone know where I might find this information?
Here's what I have so far:

1 year: paper
-Japanese word for paper is kami, which is related to spirit and folded paper represents the presence of divinity
-Ancient Egyptians thought the papyrus plant (which was used to make paper) symbolized joy, youth, freshness and love because it grew each year on the riverbanks (it's a perennial).
-Paper contains the valuable words written on it, e.g., marriage contracts, which in Judaism the contract (called a ketubah) is done by a calligrapher and then the couple hangs this on their wall.
-Can also symbolize fragility

5 years: wood
-Wood comes from tress and trees are often a symbol for life and wisdom
-Central Asia, Japan, Korea, Australia, Native Americans (and Artie) believe that trees are the ancestors of humans. Old adage 'knock on wood' might be related to this (invoking the spirit for good luck).
-Tree/wood is a solid structure. We build houses from it, etc.

10 years: Tin
-The Penguin Dictionary of Symbols associates tin with wealth, optimism, and assurance
-Tin was first used to preserve things (canned food)

15 years: Crystal
- Clarity (crystal clear)
-Spiritual truth (ok, kinda hocus-pocus...)
-People use crystals as lucky charms
-Crystals are used for healing in alternative medicine because of the energy they generate (another more relatable way to understand the "energy" is the fact that you can make music by rubbing crystal wine glaases together. sort of.)

20 years: China
-In order to make porcelain you typically use kaolin clay and feldspar, which are looked at as the yin and yang of materials.
- In China, their traditional wedding porcelain has an image of two mandarin ducks (a male and female), which are known for their fidelity to each other.

25 years: Silver
-Precious metal
-Almost every culture-Egyptians, Phoenicians, Minoans, Byzantines, Persians, Indians, Tibetans, including us, have looked to silversmiths for creating all sorts of gifts
-Alchemists used the same symbol for the moon as they did for silver (geeky, but kinda cool)