Dragonshards are hard, translucent stones similar to precious gemstones, appearing in three varieties: golden-veined Siberys shards, Khyber shards filled with veins of midnight blue to oily black and crimson-swirled Eberron shards.
Dragonshards are rarely found large enough to be crafted into items of any significant size. Most often, a small dragonshard is formed into a piece of jewellery to serve as a personal focus to enhance the power of a dragonmarked character. A larger dragonshard, up to the size of a fist, can be crafted into a larger focus item.
Unattuned dragonshards have hardness 10. The process of attunement softens a stone slightly, reducing its hardness to 8. A dragonshard has 20 hit points per inch of thickness.
Source: Eberron Campaign Setting (Page 127)