This mod aims to add real life endangered elements with behaviour based on their real world properties, I'll go through each one added so far.
Osmium is found in taiga biomes and is very dense, blocks of osmium fall, smashing the ground damaging entities and smelting a whole ore only gives you a single nugget. You can even use it to craft a new weapon with it's own unique enchantments and behaviour, the Osmium Warhammer.
Lithium is more an allegory for all group 1 elements; it's extracted from salt found in deserts, it will start fizzing before exploding if the block contacts water, is used to make bleach powder for undying things and will soon have some new redstone components.
Tungsten in real life is used to make filament bulbs it's properties are based around luminosity and while right now it adds some interesting light sources but I don't consider it finished at all and I'll add at least 2 other uses in the future :)
About why Elemental was made
This mod was created in part to spread awareness about the lack of recycling and common misuse of elements. Largely in conversation of this nature, fauna and flora conservation receive most of the attention since we can relate to the plight of animals and even trees before we can minerals but in reality their increased use might pose a greater threat to our modern way of life; lithium in phone batteries, germanium in fibre-optic cable and indium in your computer chips, these elements are all completely necessary for information technology and are running out fast. Read more about the issue here.