BuilderIcons
BuilderIcons is a Font that is used by Roblox to display things such as the icons within the Player List any various of other places.
They can also be found within images, but some icons appear to be available within that font. This font should be available in all Roblox installations under the same path.
:And if not, then someone at Roblox should start fixing that.
Fun Fact: this character here turns into the Discord Logo in this Font. I don’t know if this is a coincidence or not, because it’s a tilted cube here. (This should and is not meant to work in Roblox though. Which gives an interesting insight about Roblox’s security about unicodes.)
Edit: It is a coincidence because is the glyph that appears on all undefined unicodes for
BuilderSans.
Fonts
There’s one .json file that holds the definitions for 2 fonts, BuilderIcons-Regular and BuilderIcons-Filled when Bold.
Font.new("rbxasset://LuaPackages/Packages/_Index/BuilderIcons/BuilderIcons/BuilderIcons.json")
How to Use
Assign to .FontFace
Once assigned fonts are written in some kind of unique id’s
robuxpremiumdiamondroblox-plus
As in robux, premium diamond and roblox-plus produces the thing from the top of this post.
You can also use it with Rich Text, simply by doing this
<font family="rbxasset://LuaPackages/Packages/_Index/BuilderIcons/BuilderIcons/BuilderIcons.json">roblox-plus</font>
How to Explore Icons
They’re in the .ttf file, and I am only aware about https://fontdrop.info/ for now. If you know about other tools, let me know. Otherwise one would have to parse a .ttf by themselves.
Simply hover over an icon and you’ll see the name. The name is what you have to type into the text to display the icon.
Should I use this font?
There are various of interesting icons in there, such as a Gift Box, Controller Icons, other symbols, Backpack icons.
It is really unclear if some of these icons will stay on there or whether Roblox removes them from time to time.
If you really want one of these icons, you could export them separately and re-upload them as a picture.
It makes sense to use the font over uploading an image, because Roblox uses the font by default, so it is loaded into memory, and should be more efficient memory-wise? 
Not sure.
Will Roblox break this font in the future?
I don’t know. But I feel like they should REALLY standardize or maybe integrate this Font OFFICIALLY.
Which ever Team at Roblox owns this Font, should comment on this.
Is it safe and does it support the Roblox Filter?
So… there are these unicode characters I think. These get filtered. However, these here do not. Which is a problem for a few of the icons in there. Now do note. If your game has Rich Text enabled anyhere with Roblox’s Text Filter. In theory it shouldn’t be your fault if something doesn’t get filtered.
The bad things to watch out for would be 3rd Party Site Icons within that font, such as GitHub, YouTube, LinkedIn, Guilded, etc. these shouldn’t display to users that aren’t policy-able to view it. Most of these get filtered, but not all. Guilded didn’t get filtered by Roblox’s Text Filter for example.
Along with that, this font seems to be also triggerable by Unicode. However, Roblox doesn’t display specific unicodes from HTML Tags, so it should be fine. In many ways, regular players could already use that font in Rich Text, so it wouldn’t be your fault.
Also copy pasting unicode directly, e.g. this one will not work, so it should be fine.
