The problem is that this Trade exist in the game but the chances to get it are very low.

So, why is this Trade very rare to find?
The reason is, the chances to find this Trade are 1 out of 33, because it's settled at the Expert level of the Mason Villagers and the current Java Edition Trading System has multiple inputs in some Levels, for example on Novice level a Farmer has a variety of Trades which are:

• 20 Wheat for 1 Emerald
• 26 Potatoes for 1 Emerald
• 22 Carrots for 1 Emerald
• 15 Beetroot for 1 Emerald
• 1 Emerald for 6 Breads

The system will choose 2 Trades for the Novice from these 5 Possibilities, those 2 have equal chances since are only 5 Individual Trades.

At Mason Expert Level there's only 3 Inputs for this Level, but here is the Issue.
In 2 Inputs the Mason is offering a random variety of Blocks.
• A Random Terracotta Block out of 16, for 1 Emerald
• A Random Glazed Terracotta of course out of 16 Blocks for 1 Emerald.
• 12 Nether Quartz for 1 Emerald

So what happens is that the System takes these random Blocks as individual Inputs (Since it needs to sort a randomization) meaning that this Consist in 33 Possibilities, where the Quartz Trade is stuck against a dense majority of a same category Trade making it almost impossible to find this Trade on Expert Level, only letting the Luck decide in a Sea of very, very slow chances.

The solution for this is move this Quartz Trade to Master Level where it can compete in a more favorable Pool of Possibilities (1 out of 3) in this way making the Trade at least possible to see in a Vanilla World once.