Of course a large part of the value of gem quality diamonds is based on perception, even when the quality of manufactured diamonds exceeds that of natural ones.
This chances in 2018 when diamonds become a byproduct of various manufacturing methods. "Sideline" diamonds enjoy a brief surge in popularity and value when gems of virtually any color can be obtained.
But the value of diamonds continue to plumet. By 2020, many nations consider diamonds, diamond dust, and nano diamond saturated materials to be hazardous waste. Funding to find new uses for diamond based material doubles. Tools using diamond tips or dust become so inexpensive that they are handed out on convention floors like ink pens once were. Hand carved and machine milled art produced using "diamond tools" upon blocks of synthetic gemstone begin appearing on the market.
Blue diamonds, due to their semi-conductor uses, remain less available for gemstone use, but the increased availability and application of blue diamond to semiconductor production causes computer speeds to increase 10 fold almost over night and 100 times by the end of the decade.
In 2019 it is discovered that certain types of indigo diamonds can be used to produce highly efficient and inexpensive solar electrical panels and sheets, the oil and gas industries bankrupt themselves trying to buy up all supplies. By 2025 nearly every vehicle is flat black (running lights become mandatory in the next few years) and most communicators (the difference between smart phones and tablets having nearly disappeared by 2015) have a black panel on the back. Fracking becomes an international crime, retroactively. Most parties found guilty of promoting the process are exiled to the Mars orbital colonies.
it is discovered in 2023 that various blends of diamond dust and activated carbon can be used to selectively filter out nearly any impurity from water, while making those one time impurities available for reuse in production. Sewage treatment plants become profitable almost immediately. Water quality improves geometrically over the next 8 months. When it is found that the process works on sea water, the price of metals and other raw materials drops to nearly nothing. It is only when laws requiring that ocean salinity be protected that the price of salts jumps from almost nothing, to nearly notable. Silver, gold and diamond jewelry is sold in clamshell packaging. (In 2032 the production of clamshell packaging is banned and violators are exiled to the Mars colonies. - This prompts an extra terrestrial race calling themselves Varnir, or Saskuati'i and Yeti'i, depending on whether the individual was warm or cold weather adapted to return to Earth in 2040. Upon finding that everyone looks good in Armani, they pronounce hope that Earth may develop a civilization someday after all.)