The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

231. And He who sat there was like a jasper and a sardius stone in appearance. (4:3) This symbolizes an appearance of the Lord's Divine wisdom and love in outmost expressions.

In the Word a stone symbolizes truth in outmost expressions, and a precious stone symbolizes truth made translucent by the presence of good (no. 915).

Two colors are fundamental to all the rest in the spiritual world: the color white and the color red. The color white takes its origin from the light of the sun in heaven, thus from spiritual light, which is bright white; and the color red takes its origin from the fire of the sun there, thus from celestial light, which is flaming.

Because spiritual angels are governed by truths of wisdom from the Lord, they live in that bright white light, and are therefore attired in white; and because celestial angels are governed by goods of love from the Lord, they live in that flaming light, and are therefore attired in red. These two colors are consequently found in precious stones in heaven, where they exist in great abundance.

It is owing to this that in the Word precious stones symbolize qualities connected with either the truth of wisdom or the goodness of love; and because jasper is bright white, it symbolizes qualities connected with the truth of wisdom, while because sardius is red, it symbolizes qualities connected with the goodness of love.

These stones symbolize an appearance of Divine wisdom and Divine love in outmost expressions because all precious stones in heaven draw their origin from the outmost constituents of the Word, and they owe their translucence to the spiritual meaning of the outmost expressions in it. The reality of this may be seen in The Doctrine of the New Jerusalem Regarding the Sacred Scripture, nos. 44, 45. The outmost expressions of the Word are the truths and goods in its literal sense.

Someone in our world can hardly believe that this is the origin of precious stones in heaven, because he does not know that everything found in the spiritual world is a correspondent form, and that everything found in the natural world takes its spiritual origin from those forms. From conversation with angels I have been given to know that this is the origin of precious stones in heaven, and also to see that it is the origin with my own eyes. It is the Lord alone, however, who causes their formation.

In contrast, dark colors take their origin from hell, of which there are also two. One is the opposite of white, the kind of darkness that exists with people who have falsified the truths of the Word. The other is the opposite of red, the kind of darkness that exists with those who have adulterated the goods of the Word. The first kind of darkness is satanic, the latter diabolical.

Regarding what jasper and sardius symbolize, more may be seen in the exposition of chapter 21, verses 11, 18-20.