Gold and Silver brought with it different ways to evolve Pokemon both old and new. Many of these would return in future generations and inspire even more lucrative methods.

Stone-based evolution

Although stone-based evolution wasn't necessarily new, a new stone was indeed introduced in Gold and Silver. This would be the Sun Stone, which is only acquired by coming in first place at the Bug-Catching Content. The Sun Stone only seems to affect Grass-type Pokemon, one of whom is old and one of whom is new.

Pic	Name	Base Stats & Type						Evolution Method	Evo Pic	Evo Name	Evo Base Stats & Type
		HP	Att	Def	S.Att	S.Def	Spd				HP	Att	Def	S.Att	S.Def	Spd
	#044 Gloom							Sun Stone		#182 Bellossom
		60	65	70	85	75	40				75	80	85	90	100	50
	#191 Sunkern							Sun Stone		#192 Sunflora
		30	30	30	30	30	30				75	75	55	105	85	30

A secondary evolution stone was introduced in these games as well: the Everstone. It doesn't cause evolution though, it prevents it. It's useful for when you want to trade a Pokemon that evolves by trade. It, like the Sun Stone, is found by playing the Bug-Catching Content (though you have to come second).

Happiness-based evolution

This kind of evolution mainly affects Baby Pokemon, though it is also used for evolving Generation I Pokemon. To evolve using this method, the Pokemon must have very high happiness when it levels up. Sadly this means you can't evolve your Level 100 Golbat you brought in from Red or Blue with the Time Capsule, but it does mean you can theoretically have a Level 23 Crobat if you Golbat ingame.

Things that will jump start this process include levelling up the Pokemon, feeding it vitamins, giving it haircuts, and more. Click here for full details

Pic	Name	Base Stats & Type						Evolution Method	Evo Pic	Evo Name	Evo Base Stats & Type
		HP	Att	Def	S.Att	S.Def	Spd				HP	Att	Def	S.Att	S.Def	Spd
	#042 Golbat							Happiness		#169 Crobat
		75	80	70	65	75	90				85	90	80	70	80	130
	#113 Chansey							Happiness		#242 Blissey
		250	5	5	35	105	50				255	10	10	75	135	55
	#133 Eevee							Happiness Daytime Only		#196 Espeon
		55	55	50	45	65	55				65	65	60	130	95	110
	#133 Eevee							Happiness Night Only		#197 Umbreon
		55	55	50	45	65	55				95	65	110	60	130	65
	#172 Pichu							Happiness		#025 Pikachu
		20	40	15	35	35	60				35	55	30	50	40	90
	#173 Cleffa							Happiness		#035 Clefairy
		50	25	28	45	55	15				70	45	48	60	65	35
	#174 Igglybuff							Happiness		#039 Jigglypuff
		90	30	15	40	20	15				115	45	20	45	25	20
	#175 Togepi							Happiness		#176 Togetic
		35	20	65	40	65	20				55	40	85	80	105	40

Time-based evolution

This evolution method only affects one Pokemon: Eevee. It requires the same conditions to be satisfied as Happiness-based evolution, but the resulting Pokemon will be different based on whether it is morning and day or night.

Pic	Name	Base Stats & Type						Evolution Method	Evo Pic	Evo Name	Evo Base Stats & Type
		HP	Att	Def	S.Att	S.Def	Spd				HP	Att	Def	S.Att	S.Def	Spd
	#133 Eevee							Happiness Daytime Only		#196 Espeon
		55	55	50	45	65	55				65	65	60	130	95	110
	#133 Eevee							Happiness Night Only		#197 Umbreon
		55	55	50	45	65	55				95	65	110	60	130	65

Item trade-based evolution

One of the most common forms of evolution introduced was Item trade-based evolution. This method works the same way as trading with the exception being that the Pokemon must be holding an item to evolve. Trading via Time Capsule to older versions will not get you a Steelix in Red though, so keep that in mind. Often times the items you need only appear once in the overworld and are rarely found on wild Pokemon so don't make the decision lightly.

Pic	Name	Base Stats & Type						Evolution Method	Evo Pic	Evo Name	Evo Base Stats & Type
		HP	Att	Def	S.Att	S.Def	Spd				HP	Att	Def	S.Att	S.Def	Spd
	#061 Poliwhirl							Trade with King's Rock attached		#186 Politoed
		65	65	65	50	50	90				90	75	75	90	100	70
	#079 Slowpoke							Trade with King's Rock attached		#199 Slowking
		90	65	65	40	40	15				95	75	80	100	110	30
	#095 Onix							Trade with Metal Coat attached		#208 Steelix
		35	45	160	30	45	70				75	85	200	55	65	30
	#123 Scyther							Trade with Metal Coat attached		#212 Scizor
		70	110	80	55	80	105				70	130	100	55	80	65
	#117 Seadra							Trade with Dragon Scale attached		#230 Kingdra
		55	65	95	95	45	85				75	95	95	95	95	85
	#137 Porygon							Trade with Up-Grade attached		#233 Porygon2
		65	60	70	85	75	40				85	80	90	105	95	60

Stat-based evolution

This is unique to Tyrogue. If, by Level 20, Attack is higher than Defense, you'll get Hitmonlee. If Defense is higher than Attack, you'll get Hitmonchan, and if both are equal, you'll get Hitmontop.

Pic	Name	Base Stats & Type						Evolution Method	Evo Pic	Evo Name	Evo Base Stats & Type
		HP	Att	Def	S.Att	S.Def	Spd				HP	Att	Def	S.Att	S.Def	Spd
	#236 Tyrogue							Level 20 Attack < Defence		#107 Hitmonchan
		35	35	35	35	35	35				50	105	79	35	110	76
	#236 Tyrogue							Level 20 Attack > Defence		#106 Hitmonlee
		35	35	35	35	35	35				50	120	53	35	110	87
	#236 Tyrogue							Level 20 Attack = Defence		#237 Hitmontop
		35	35	35	35	35	35				50	95	95	35	110	70