You have to give those involved with Jem credit for the mind boggling array of fashions shown on the cartoon. They were nearly always consistent in look and who wore them, not to mention that each main character (there were up to 10 main characters per show!) changed an average of three times per episode, quite often more regularly than that. I'm not sure if there's another cartoon out there that rivals Jem for the sheer diversity of fashions depicted on screen (I'm thinking maybe Sailor Moon, or another anime may come close).
Whether any of these outfits were intended to be produced as toys or not is conjecture.  At this point, I simply do not know.  Unfortunately I also cannot say who designed these, or any of the other Jem fashions, but most of the cartoon-only fashions I would guess were designed by Sunbow animators/designers, not the Hasbro toy designers.  I am led to believe the people responsible for the cartoon-only fashion designs are Debbie Pugh and Paula La Fond. So a huge kudos for their inspiration and talent!  Please note, the episode order for Jem has been contested; I am using the Rhino DVD order here - it's not of much interest to me to debate on wether it's the "right" order or not!

Model Sheet Numbers
It looks like most, if not all, fashions worn on the show had a model sheet number.  The numbers are generally in the same order they appeared on the show, going by the Sunbow production codes, not the airing order.  For instance, Aja's stage & doll fashion has the Model Sheet number of: "A-1", the "A" standing for Aja, the 1 being the "name" of the fashion.  Later fashions, such as Aja Alaska Sweater, have higher numbers (A-75-43).  The second number corresponds to the Sunbow production code for the episode, which is "Middle of Nowhere".  The fashions that were limited to videos (such as Aja Mermaid)  had "Aja Mermaid 1505-15 Song 1" or a similar designation on the model sheet and do not usually appear outside of that specific song.  Unfortunately as I haven't seen all of the model sheets, I don't know if every single fashion was catalogued- for example, Aja's funeral dress which appears on screen for all of 4 seconds- but reoccurring fashions, Hasbro-designed or not,  most certainly would be catalogued in this system.  I'm still learning as we uncover more of these model sheets; I hope one day I'll have the "official" numbers for all of these fashions!

By Episode
Look at each cartoon-only outfit as it is introduced to the series.  You'll see a surprising cohesiveness to the fashions.

By Character
View each cartoon-only outfit by character.  See how their wardrobe changes, what are popular colours/styles for them, etc.

By Episode- You'll notice a certain logic in which episodes went where- each "batch" starting with a 2 or 3 parter by Christy Marx, the head writer.  You'll also see where the art changes dramatically (the first five episodes compared to the Starbright episodes), when they started recycling songs & using "B" team (a Korean studio, as opposed to the Japanese studio) animation ("season 3")  I believe what happened was that the first 5 shows listed here (Beginning through Battle of the Bands) were not turned into episodes until the series was picked up after the success of the Super Saturday 7 minute clips- which explains the unusual production codes that Sunbow assigned them.  Don't ask me why Jetta/Raya episodes have earlier production codes than the Talent Search episodes.

Please note this section is still under major construction (ie: the links in "bold" font), it will be some time before it is complete!

Season 1:  Episodes comprised of the Super Saturday 7 minute clips

1.  The Beginning (Sunbow #008)
2.  Disaster (Sunbow #009)
3.  Kimber's Rebellion (Sunbow #010)
4.  Frame Up (Sunbow #011)
5.  The Battle of the Bands (Sunbow #012)

Season 2

6.  Starbright Part 1: Falling Star (Sunbow #001)
7.  Starbright Part 2: Colliding Stars (Sunbow #002)
8.  Starbright Part 3: Rising Star (Sunbow #003)
9.  The World Hunger Shindig (Sunbow #004)
10. Adventure in China (Sunbow #005)
11. Last Resorts (Sunbow #006)
12. In Stitches (Sunbow #007)
13. The Music Awards Pt. 1 (Sunbow #014)
14. The Music Awards Pt. 2 (Sunbow #015)
15. The Rock Fashion Book (Sunbow #013)
16. Broadway Magic (Sunbow #016)
17. In Search of the Stolen Album (Sunbow #017)
18. Hot Time in Hawaii (Sunbow #018)
19. The Princess and the Singer (Sunbow #019)
20. Island of Deception (Sunbow #020)
21. Old Meets New (Sunbow #021)
22. Intrigue at the Indy 500 (Sunbow #22)
23. The Jem Jam Part 1 (Sunbow #23)
24. The Jem Jam Part 2 (Sunbow #24)
25. Culture Clash  (Sunbow #25)
26. Glitter And Gold  (Sunbow #26)

Season 3.

The Talent Search Part 1  (Sunbow #30)
The Talent Search Part 2  (Sunbow #31)
Scandal  (Sunbow #28)
One Jem Too Many  (Sunbow #38)
The Bands Break Up  (Sunbow #34)
The Fan  (Sunbow #36) 
Father's Day  (Sunbow #40)
The Treasure Hunt  (Sunbow #32)
Aztec Enchantment  (Sunbow #45)
Music is Magic  (Sunbow #39)
The Jazz Player (Sunbow #41)
Danse Time  (Sunbow #35)
Roxy Rumbles  (Sunbow #44)
Alone Again  (Sunbow #49)
KJem  (Sunbow #46)
Trick or Techrat  (Sunbow #33)
The Presidential Dilemma  (Sunbow #29)
Rock N' Roll Express  (Sunbow #42)
The Mardi Gras  (Sunbow #50)
The Middle of Nowhere (Sunbow #43)
Renaissance Woman
Journey to Shangri-La
Journey Through Time
Britrock
Out of the Past
Hollywood Jem Part 1: "For Your Consideration..."
Hollywood Jem Part 2: "And the Winner Is..."
The Stingers Hit Town Part 1
The Stingers Hit Town Part 2
Video Wars
Beauty and the Rock Promoter
Homeland, Heartland
Midsummer Night's Madness
The Day the Music Died
That Old Houdini Magic  (Sunbow #62)
Straight From the Heart  (Sunbow #63)
A Change of Heart
Riot's Hope
A Father Should Be...

Opening Credits: I didn't want these at the beginning because they don't really "go" with the rest of the show, but there are a few outfit differences that are pretty interesting!

By Character (this section will be behind the show wardrobes until I'm finished)

Aja

Episodes 1-10
Episodes 11-16
Episodes 16-26
Episodes 26-44
Episodes 44-65

Episodes 6-64

Episodes 14-58

Page 1 - Episodes 1-6
Page 2 - Episodes 7-15
Page 3 - Episodes 15-21
Page 4 - Episodes 22-36
Page 5 - Episodes 37-57
Page 6 - Episodes 59-64

Episodes 1-10
Episodes 11-33
Episodes 40-

Episodes 27-43
Episodes 44-65

Episodes 1-9
Episodes 10-16
Episodes 16-22
Episodes 25-37
Episodes 38-

Episodes 54-64

Episodes 1-11
Episodes 11-21
Episodes 21-43
Episodes 43-65

Episodes 54-63

Episodes 27-35
Episodes 35-48
Episodes 48-65

Episodes 1-16
Episodes 20-65

Episodes 54-63

Episodes 1-12
Episodes 13-23
Episodes 23-46
Episodes 46-

Episodes 1-10
Episodes 11-16
Episodes 16-25
Episodes 25-38
Episodes 42-

Episodes 1-12
Episodes 13-23
Episodes 23-
Episodes 46-end

Episodes 6-56

Thanks very much to Walter, whose production model sheets have been an inspiration (and some I've actually used here, coloured in).  As well, some of the faces have been taken from his production sheets as well, the rest (vast majority) outfits have been hand drawn by me.  *Please* ask me for permission (or just credit me as Pranceatron) if you want to use my drawings!
Thanks also to Sarah & Tiffany for various emails and fine tuning on this page.  So nice to have people double checking your work for you ;)!