“‘Time, heat and pressure.  The same things that make a diamond also make a waffle.’ ~Scott Meyer” was posted on the newsgroup Google Groups: seattle.eats on June 14, 2001. American artist and author Scott Meyer used the saying in his webcomic

in 2007 and 2011, but beginning “Heat, pressure and time.”
         
“Heat, Pressure, and Time. The three things that make a diamond, are also the three things that make a waffle” was posted Reddit—Shower Thoughts on July 11, 2016.
 
   
Wikipedia: Scott Meyer
Scott Oscar Meyer is an American author, comedian, and artist, known for his webcomic Basic Instructions and his comic fantasy series Magic 2.0.
 
Google Groups: seattle.eats
Suggestion for a great Seafood restaurant?
Comedy Guru
6/14/01
(...)
“Time, heat and pressure.  The same things that make a diamond also make a waffle.”
~Scott Meyer
 
Google Groups: alt.comedy.standup
I need a name for my business
Comedy Guru
6/18/01
(...)
“Time, heat and pressure.  The same things that make a diamond also make a waffle.”
~Scott Meyer
 
Google Books
The Manual of How to Cheat on Your Wife
Edited by Grace Ann Neff
Virginia Beach, VA: Dead Dog Publishing
2002
Pg. 97:
Time, heat and pressure; the same things that make a diamond also make a waffle.
   
Twitter
Brian Webster
@bwebster
QOTD: “Heat, pressure, and time.  The three things that make a diamond, also make a waffle.” http://tinyurl.com/2wzzw8
9:57 AM - 9 Sep 2007
 
Twitter
Keith
@SELLTEAM
Heat, pressure and time are three things that make a diamond. That is also three things that make a waffle.
12:41 AM - 5 Aug 2013
   
Reddit—Shower Thoughts
Heat, Pressure, and Time. The three things that make a diamond, are also the three things that make a waffle.
submitted July 11, 2016 by IDontCareAboutUpvote
COMMENTS
hillip1
This is from a comic. http://basicinstructions.net/basic-instructions/2011/9/29/how-to-decide-what-to-do.html
 
Don_Bardo
Stolen from Scott Meyer’s Basic Instructions Volume 1: Help is on the way!.
Edit: The original strip
 
Twitter
ｊｏｊｉ
@otphrhnz
Heat, Pressure, and Time. The three things that make a diamond, are also the three things that make a waffle.
12:58 PM - 11 Jul 2016
     
Reddit—Shower Thoughts
Heat, Pressure and Time. The three things that make a Diamond, are also three things that make a waffle
submitted October 1, 2016 by Ask-if-I-Like-Lemons
 
Reddit—Shower Thoughts
Heat, pressure, and time. The three things that make a diamond, are also the three things make a waffle.
submitted February 4, 2018 by jcd718
 
Reddit—Shower Thoughts
Heat, Pressure, and Time. The three things that make a diamond, are also the three things that make a waffle.
submitted July 14, 2018 by ebw15