More Awesome Than You!

Do they spawn in random places or exact places that can be cataloged? So far I have failed at remembering exactly where I have found something before and I'm wondering if its worth it to keep notes.

Seems like certain item types are more likely to show up in certain areas. Seeds around the parks/in woods, gems and ores around the mine and Stonehenge (and, for some reason, the criminal warehouse), not sure about moon rocks since none of my Sims collect them.

I'm assuming there's some sort of method involved. A Luminorous (sic) Gem spawns appears around the path behind my house every couple of days; Stonehenge is always infested with bugs; the mine attracts lots of metals. At the very least, types of collectables (seeds, metals, bugs) appear in constant places although which one actually drops pops up seems to vary.

There are some space rocks up by the waterfall, as well as some gems (I think, tanzenite?).  There are also some bell pepper plants and apple trees (and probably more) up there.  I know there were some rocks/gems at the graveyard, but I can't remember what kind.  My sim also got one rare seed and two special seeds just outside the graveyard, but hasn't been able to plant them yet.  There are also gems and space rocks and metals on the beach.

There's definitely places where certain kids of things always spawn.
I would guess that:
1. The varieties are weighted so that the more common ones spawn the most,
2. The rare varieties are on a timer, or
3. That certain common varieties can randomly spawn as a rare variety.  For example, maybe a spot that's usually a topaz has a certain percentage chance to spawn as a pink diamond instead.  

Or some combo of all three.  

If you check with the Collection Helper (get ye some, it's an inventory item so you can pass it around), you'll see that seeds are absolutely all over the place. Bugs tend to congregate around Stonehenge, the mine, moths in the graveyard, one always in central park, spottily around the beaches, and then in the middle of nowhere. Gems/metals are most prevalent around the mine (metals, mostly), some in Stonehenge, a lot around the parks/fishing holes/beaches, and some just randomly strewn about.

The catacombs can be a good place to get rares, too. One foray recently gave me an unknown special seed, a magical gnome, and a pink diamond.

Oh I'm so glad you mentioned that the Collection Helper is an item.  I almost got it for my founder sim but she had one foot in the grave at that point and I didn't want to blow all those points on it when she wasn't even the collector of the fambly. 

I have yet to find the catacombs - any pointers as to where they are?

It's the building at the graveyard where you manage the tombstones of the sims the game finds important and where you sign up for graveyard part time jobs.

I'm certain the method is similar to WoW (among other similarities - see item quality text color).
To be exact, the spawn points for EVERY item is fixed, it's the type of item that spawns that's random. Each spawn point has a list of potential items. When you collect an item, a timer in the spawn point goes off, and when it's completed, a new item selected randomly from the particular spawn point's item list, appears. For example, seed spawn points at the central park have common seeds in their list, apple, grape, etc, while the seeds in the lists of the points around the science facility are of "special" rarity.

There are a few seeds around the grocery store. I found a couple of 'unknown' seeds there. Took forever for them to grow, and when they did, one was a bad quality onion plant, and the other was watermelon.

Yeah you find the gems, get them cut, display them, and then what? Do they affect mood, maybe?

They increase a room's environmental score.

I always find plenty of seeds near the woods by the Science Center.  That's where I've found several money tree seeds.  The random seeds are always in the same place, so respawn after a certain amount of time.  I think I found the Life Plant and Death Flower bush seeds there too.  I've figured out what to do with the Life Fruit, but haven't figured out the Death flower bush.