Over the years, Sirens have worked with many different species, learning the best way to exchange and show information to a host. The triangle that is usually resting within Sapphira's collar is one of these items in question.
When a suitable energy source is transferred into the gem, such as magic, it projects a screen. Using thought, touch, or electrical signals, the screen can be made to show different information stored upon the gem onto the screen in an interact-able manner. These images can be moving or static, though they cannot produce sound. The data itself is transferred to the gem by electrical impulses, similar to those which activate it's use.
The best way to explain it's use to humans is that its similar to a computer screen, with the gem storing information like an SSD. The amount of data it can store is vast, and mostly unknown. Sirens are not too bothered to try and find its limits.
Sirens are not the only species to be able to use this item, though it was with their help that it was created. Any creature that can send a suitable energy source into the gem, along with controlled electrical signals for usage could use these items. A human's nerve impulses would be suitable for the electrical signals, though they would need to plug it in to a larger electrical source in order to power it, such as a car battery.
Sirens tend to use this item for all information purposes, and nearly every one known carries some form of it. For some it is used as an identity chip, and is generally accepted as a galactic passport.
These items come in many shapes and sizes; most are small and placed into jewelry for easy carry. At their natural size they can project an A4 sized screen. They extend out to a larger size when larger screens are needed. More energy is needed to power the gems as the screens get larger.

More world building for the Sirens. There's a reason Sapphira tends to keep her collar hidden under a scarf; it would cause her a tonne of trouble if her gem was stolen.
...I'll also update this when the item actually has a proper name, rather than 'gem'.
Due to the magical usage of the gem, it can be used in both past and present day roleplays, including those with high tech worlds. However, these worlds must be magical in nature for Sirens to really be able to work as a species. I am attempting to make it so that Sirens can easily slip into another person's world while still being their own species in themselves.

This outfit design and especially the colour scheme is normal wear for Sapphira. This version is the...more open look. Soon I'll have a reference sheet up and ready to show you of her everyday wear.

Sapphira belongs to me.
Artwork done by Charmrage
There is a version without the screen so that the outfit can be easily seen for those looking to draw her in it! [Thank you again, Charmrage!]