Does Flame Blade count as a weapon for Green-Flame Blade?

Question

The description for the green-flame blade spell (SCAG, p. 143) says:

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range.

It doesn't specify that the "attack with a weapon" can't be a "spell attack with a weapon".

Does the effect conjured by flame blade count as a weapon or not and therefore be used in conjunction with green-flame blade?

It doesn't say in the spell description of flame blade. It only states it is similar in size and shape to a scimitar which sounds strikingly like a weapon to me.

Answer 1

No

In fact, you can't even use a Flame Blade spell as part of an Attack (action).

The spell states:

You evoke a fiery blade in your free hand. The blade is similar in size and shape to a scimitar, and it lasts for the duration. lf you let go of the blade, it disappears, but you can evoke the blade again as a bonus action.

You can use your action to make a melee spell attack with the fiery blade. On a hit, the target takes 3d6 fire damage.

The only mechanical uses of the the fiery blade are its unique action (make a melee spell attack, deal 3d6 fire damage on hit), and its ability to shed light.

Since you can't make a melee weapon attack with it, it isn't a weapon, and thus can't be used in conjuction with Green-Flame Blade.

Compare this to the spell Shadow Blade (from Xanathar's) which says:

You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60)

The spell description specifically states that the effect counts as a weapon, and it defines the weapons properties.

Answer 2

You can't combine the spells because you only have one action on your turn.

Although GreySage's answer excellently explains why the fiery blade produced by flame blade does not count as a weapon usable as part of casting green-flame blade, there is an additional and substantial reason you cannot combine these two spells: the action economy.

The text for green-flame blade explains that you must make an attack as part of the action used to cast it (the Cast a Spell action).

As part of the action used to cast this spell, you must make a melee attack with a weapon...

The text for flame blade explains that using the fiery blade it creates requires you to use an action to make the attack (this special action doesn't have a name--it is granted by the spell effect).

You can use your action to make a melee spell attack with the fiery blade.

If you've already used your action on your turn to use the Cast a Spell action, you can't also use the special action granted by flame blade, since you only have one action on your turn. (Even if you're a Fighter using Action Surge, the actions are resolved separately, so an attack made as part of Cast a Spell can't also somehow be the same attack made as part of the special action of flame blade.)