Well I've been doing well on fire giants today, two rune scimmys, and apparently a longbone. What to do with it?

I think they're used in Construction.

Burying it will give the same exp as Big Bones but I suggest you bring it to the goblin in Dorgesh Khaan to get Construction exp instead. His name is Balrak(?)?? :?

 

 

 

 

 

 

 

EDIT: From Tipit's Item Database -----"There are two uses. Bury this for 15 prayer experience, or sell it to Barlak in Dorgesh-Kaan for construction exp and gold.

 

Notes Barlak will reward you with 1000gp and 1500xp in construction for each bone. However, you must be level 30 construction and have a house to get the experience. Barlak is at the north end of the corridor located southwest of the marketplace in Dorgesh-Kaan. You must have completed Death to the Dorgeshuun to visit Barlak. See mini map."

people need to learn how to use the database...

 

 

 

http://tip.it/runescape/index.php?rs2item_id=6799

Indeed, a long bone gives 1500 construction xp and 1k gp if you give it to Barlak. Every monster that drops big bones 100% of the times can drop a long bone. There is also a curved bone, which is dropped by 100% big bones dropping monsters too, but it's a bit rarer than a long bone. A curved bone gives 2.5k gp and 2250 construction xp if you give it to barlak.