Gems
From the RuneScape Wiki, the wiki for all things RuneScape
Gems also are a frequent drop from a wide range of monsters and are occasionally given out as gifts from various Random events. Random events and monster drops generally give sapphire, emerald, ruby or a diamond. Dragonstones are dropped only by some of the higher level monsters, although any monster that drops gems and many random events, can drop or give Half of a keys to unlock a chest in Taverley which contents include uncut dragonstone.
Gems can also be mined from the Gem Rock in the northwest corner of Shilo Village, Lunar Isle Mine and occasionally in Tai Bwo Wannai Cleanup minigame. Gem rocks contain, in order of highest probability, opal, jade, red topaz, sapphire, emerald, ruby and diamond.
Some gems can be purchased (or stolen) or sold to a gem shop identified with a red gem 
icon on the map.
The original free version gems were officially released in the crafting update in May 2001. In March 2001, Jagex made gems appear while mining, although it was unintentional.
Ironically, uncut gems are worth more on the player market than cut gems are as players are able to get more Crafting experience from cutting gems.
Because of the high price of uncut gems, it is usually worth more when mining to keep the gems you get because an uncut sapphire sells for more than a coal.
